Re: [GENERAL] DISTINCT vs GROUP BY - was Re: is (not) distinct from
Hi David, On Sat, 4 Mar 2017 02:32:48 +1300, David Rowley wrote: >On 3 March 2017 at 18:26, George Neuner wrote: >> I know most people here don't pay much - or any - attention to >> SQLServer, however there was an interesting article recently regarding >> significant performance differences between DISTINCT and GROUP BY as >> used to remove duplicates. >> >> https://sqlperformance.com/2017/01/t-sql-queries/surprises-assumptions-group-by-distinct >> >> >> Now I'm wondering if something similar might be lurking in Postgresql? > >Yes things lurk there in PostgreSQL too. But to be honest I find the >examples in the URL you included a bit strange. There's almost >certainly going to be a table called "orders" that you'd use for the >outer part of the query. In that case the orderid would already be >unique. To do the same in PostgreSQL you'd just use: select orderid, >string_agg(description,'|') from orderitems group by orderid; assuming >all orders had at least one line, you'd get the same result. The author mentioned at the beginning that the simple queries: SELECT DISTINCT Description FROM Sales.OrderLines SELECT Description FROM Sales.OrderLines GROUP BY Description; wouldn't display the subject behavior. Of course, analyzing the much more complex queries is much more difficult. It begs the question: what actually is going on there? But I don't use SQL Server ... my interest is in how Postgresql deals with a similar situation. >Assuming that parts.partcode is the PRIMARY KEY of parts, this query >is legal in PostgreSQL. In some other databases, and I believe SQL >Server might be one of them, you would have been forced to include >part.description in the GROUP BY clause. Since PostgreSQL 9.6, if >you'd have done the same with that, internally the database would >ignore the parts.description in the GROUP BY clause, as its smart >enough to know that including parts.description in the clause is not >going to change anything as the description is always the same for >each parts.partcode, and no two records can share the same partcode. > >There's no such optimisation when it comes to DISTINCT. In PostgreSQL >as of today DISTINCT is a bit naive, and will just uniquify the >results on each column in the select clause. Although quite possibly >the same optimisation could apply to DISTINCT too, just nobody has >thought to add it yet. > >In short, the main difference is going to be the fewer columns you're >using to identify the groups the better. If you included all of the >columns in the GROUP BY clause as you put in the select list with the >DISTINCT query then in most cases the performance would be the same. I >believe the only exception to this is in regards to parallel query, as >currently only GROUP BYs may be parallelised, not DISTINCT. > >Historically with older now unsupported versions of PostgreSQL (pre >8.4) you may have also preferred to use GROUP BY over DISTINCT as >GROUP BY could be implemented internally by sorting or hashing the >results, whereas DISTINCT used to only be implemented by Sorting the >results. Although this has long since been the case. I often have occasion to use multiple mapping relations: e.g., A{1}->B{N} C{1}->B{N} together in a query where C is provided and I need to find the corresponding A(s). Frequently these queries result in the same A being found multiple times. Although the mapping tuples are small [usually just a pair of keys], the number of rows in the mapping tables may be very large, and a given query may need to join/work its way through several such mappings. Typically in such situations, I divide the query using CTEs and (try to) minimize the volume of data at each step by filtering duplicates from any results that might include them. I have always used DISTINCT to filter duplication, reserving GROUP BY for aggregations (counting, etc.). But if I understand correctly, you are saying that GROUP BY should be preferred even for the simpler use. George -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Querying JSON Lists
On 03/03/2017 11:50 AM, Sven R. Kunze wrote: On 03.03.2017 16:05, Adrian Klaver wrote: https://www.postgresql.org/docs/9.6/static/functions-json.html As to why it works on JSON arrays: Table 9-43. Additional jsonb Operators " ? text Does the string exist as a top-level key within the JSON value? " So to be picky it not does call out JSON object it says JSON value. And right above the table: " For a full description of jsonb containment and existence semantics, see Section 8.14.3. Section 8.14.4 describes how these operators can be used to effectively index jsonb data." As to how that behavior was decided on I have no idea, it just is. I think it would even be possible to add the integer-variant of the ? operator. Something I learned right now: integers cannot be object keys in json. On the flip side, they can be array elements. So, I can see a certain logic because of a uncertainty of integers. Python differs here from PostgreSQL: json.dumps({4: '34'}) '{"4": "34"}' # select '{4:4}'::jsonb; ERROR: invalid input syntax for type json LINE 1: select '{4:4}'::jsonb; ^ DETAIL: Expected string or "}", but found "4". CONTEXT: JSON data, line 1: {4... Python wraps it up, PostgreSQL fails loudly. Not that PostgreSQL is With the caveat: https://docs.python.org/3/library/json.html#py-to-json-table " Note Keys in key/value pairs of JSON are always of the type str. When a dictionary is converted into JSON, all the keys of the dictionary are coerced to strings. As a result of this, if a dictionary is converted into JSON and then back into a dictionary, the dictionary may not equal the original one. That is, loads(dumps(x)) != x if x has non-string keys. " I know because it's bit me. I use Python and I get a lot done with it, but it has its inconsistencies also: In [11]: d = {1: 'one', 2: 'two'} In [12]: dict(**d) --- TypeError Traceback (most recent call last) in () > 1 dict(**d) TypeError: keyword arguments must be strings In [13]: d = {'1': 'one', '2': 'two'} In [14]: dict(**d) Out[14]: {'1': 'one', '2': 'two'} confused by Python, but it's weird when ? operator works on both keys and arrays with text but works only on arrays with integers. So, I guess no support for integers for now might have seen like a good idea. Though there looks to be some implicit casting going on: test=> select '["12","34","45"]'::jsonb @> '"12"'::text; ERROR: operator does not exist: jsonb @> text LINE 1: select '["12","34","45"]'::jsonb @> '"12"'::text; to get '"12"' to be '"12"'::jsonb. As to why, I don't know. This makes sense to me at least, as we test structural json containment. So, testing json to be contained by other json requires it to be json. :) The confusing fact is that one can omit the array brackets in case of a single primitive value. Don't get me wrong. I don't complain as it's a usability feature. However I didn't expect it to be there in the first place and adding some brackets wouldn't hurt IMO. I'd rather consider brackets a readability support such as "this is json". It can be there if you want to maintain readability in your code: test=> select '["12","34","45"]'::jsonb @> '["12"]'; ?column? -- t or you can use an explicit cast: test=> select '["12","34","45"]'::jsonb @> '"12"'::jsonb; ?column? -- t Sven -- Adrian Klaver adrian.kla...@aklaver.com -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Querying JSON Lists
On 03.03.2017 16:05, Adrian Klaver wrote: https://www.postgresql.org/docs/9.6/static/functions-json.html As to why it works on JSON arrays: Table 9-43. Additional jsonb Operators " ? text Does the string exist as a top-level key within the JSON value? " So to be picky it not does call out JSON object it says JSON value. And right above the table: " For a full description of jsonb containment and existence semantics, see Section 8.14.3. Section 8.14.4 describes how these operators can be used to effectively index jsonb data." As to how that behavior was decided on I have no idea, it just is. I think it would even be possible to add the integer-variant of the ? operator. Something I learned right now: integers cannot be object keys in json. On the flip side, they can be array elements. So, I can see a certain logic because of a uncertainty of integers. Python differs here from PostgreSQL: >>> json.dumps({4: '34'}) '{"4": "34"}' # select '{4:4}'::jsonb; ERROR: invalid input syntax for type json LINE 1: select '{4:4}'::jsonb; ^ DETAIL: Expected string or "}", but found "4". CONTEXT: JSON data, line 1: {4... Python wraps it up, PostgreSQL fails loudly. Not that PostgreSQL is confused by Python, but it's weird when ? operator works on both keys and arrays with text but works only on arrays with integers. So, I guess no support for integers for now might have seen like a good idea. Though there looks to be some implicit casting going on: test=> select '["12","34","45"]'::jsonb @> '"12"'::text; ERROR: operator does not exist: jsonb @> text LINE 1: select '["12","34","45"]'::jsonb @> '"12"'::text; to get '"12"' to be '"12"'::jsonb. As to why, I don't know. This makes sense to me at least, as we test structural json containment. So, testing json to be contained by other json requires it to be json. :) The confusing fact is that one can omit the array brackets in case of a single primitive value. Don't get me wrong. I don't complain as it's a usability feature. However I didn't expect it to be there in the first place and adding some brackets wouldn't hurt IMO. I'd rather consider brackets a readability support such as "this is json". Sven
Re: [GENERAL] odd optimizer result, index condition "is not null" on column defined as "not null"
On 03/03/2017 17:33, Tom Lane wrote: Martin F writes: The select with filter choose an IMHO better plan Index Only Scan using tbl_foo_date on public.tbl_foo But the bigger picture here, which would become more obvious if you were working with a non-toy amount of data, is that you're asking the planner to choose between two bad options. I agree "better" was the wrong term. "different" And yes they are both bad. And in fact after some more research, I think I found https://wiki.postgresql.org/wiki/Index-only_scans#What_types_of_queries_may_be_satisfied_by_an_index-only_scan.3F which explains why the aggregate-with-filter is potentially much worse (as it accesses more rows) Lets see, if I am closer to a correct understanding. Lets see if my following assumptions are (closer to being) correct: So the real missing feature here is the opposite of what I expected. select min(id) filter(where created_at >= '2017-01-15') from tbl_foo is NOT rewritten to select id from tbl_foo where created_at >= '2017-01-15' and id is not null order by id limit 1 That is the filter is currently not transformed to a where. On the other hand, looking at the explain of explain analyze verbose select min(id) filter(where created_at >= '2017-01-15') from tbl_foo; QUERY PLAN --- Aggregate (cost=13.28..13.29 rows=1 width=16) (actual time=0.799..0.804 rows=1 loops=1) Output: min(id) FILTER (WHERE (created_at >= '2017-01-15 00:00:00'::timestamp without time zone)) -> Index Only Scan using tbl_foo_id_date on public.tbl_foo (cost=0.14..13.00 rows=57 width=16) (actual time=0.024..0.437 rows=57 loops=1) Output: created_at, id Heap Fetches: 57 Planning time: 0.080 ms Execution time: 0.901 ms 57 heap fetches, so one for every row. It seems that min() does a heap fetch for every row, even if the value for that row is bigger than the current aggregated value. That is the heap fetch happens, even if the value's visibility does not matter / the value will be discarded anyway. (Of course that is because the function can not affect the scanners decision if a row is required or not) Are my above observation and conclusions correct, or am I missing something crucially (apart from that I am only looking at a tiny fraction of reality) If you are concerned about the performance of this specific query shape, what you actually want is an index on (id, created_at). That allows stopping at the first index entry satisfying the created_at condition, knowing that it must have the min id value that does so. Thanks indeed, taking in account the true nature of "index only", the above is a good idea. regards Martin -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] odd optimizer result, index condition "is not null" on column defined as "not null"
On 03/03/2017 17:33, Tom Lane wrote: Martin F writes: Index Cond: (tbl_foo.id IS NOT NULL) only "id" is the pk, and declared "not null". So why this index condition? You're right that we could observe that the NOT NULL is implied by a table constraint and drop it, but it seems unlikely to be worth the planner cycles to do so; the condition doesn't cost much at runtime. (Also, plans that depend on table constraints for correctness have added bookkeeping costs from tracking such dependency.) Thanks for the explanation. I looked at more example and yet found another case. The planer can choose an index, where the index has the same condition as (part of) the query conditions. I added the 2 indexes create index tbl_foo_ID_1 on tbl_foo using btree (id) where (id <>1); create index tbl_foo_ID_null on tbl_foo using btree (id) where (id is not null); and used the 2 queries (already transformed, so the first does not have the "not null") explain analyze verbose select id from tbl_foo where created_at >= '2017-01-15' and id <>1 order by id limit 1; explain analyze verbose select id from tbl_foo where created_at >= '2017-01-15' and id is not null order by id limit 1; both choose the index with the matching condition ... Index Scan using tbl_foo_id_1 ... Index Scan using tbl_foo_id_null The "<> 1" condition does not appear in the plan (as it is covered by the chosen index) But the "is not null condition is kept, why? Yes I understand, it makes probably little difference in the end, but I think it is somewhat curious. This also happens, if I change id to: id bigserial (that is make in null-able / yet the index remains filtered to none null only) explain analyze verbose select id from tbl_foo where created_at >= '2017-01-15' and id <>1 order by id limit 1; QUERY PLAN - Limit (cost=0.14..0.45 rows=1 width=8) (actual time=0.039..0.044 rows=1 loops=1) Output: id -> Index Scan using tbl_foo_id_1 on public.tbl_foo (cost=0.14..13.26 rows=42 width=8) (actual time=0.026..0.026 rows=1 loops=1) Output: id Filter: (tbl_foo.created_at >= '2017-01-15 00:00:00'::timestamp without time zone) Rows Removed by Filter: 13 Planning time: 0.162 ms Execution time: 0.087 ms (8 rows) explain analyze verbose select id from tbl_foo where created_at >= '2017-01-15' and id is not null order by id limit 1; QUERY PLAN Limit (cost=0.14..0.45 rows=1 width=8) (actual time=0.042..0.047 rows=1 loops=1) Output: id -> Index Scan using tbl_foo_id_null on public.tbl_foo (cost=0.14..13.28 rows=43 width=8) (actual time=0.029..0.029 rows=1 loops=1) Output: id Index Cond: (tbl_foo.id IS NOT NULL) Filter: (tbl_foo.created_at >= '2017-01-15 00:00:00'::timestamp without time zone) Rows Removed by Filter: 14 Planning time: 0.129 ms Execution time: 0.096 ms -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Understanding pg_last_xlog_receive_location
Thanks Michael- That was indeed the issue. We have a very complex wrapper application that walks the server through multiple state transitions, and it turned out that in the state I was running the query from, streaming replication wasn't configured. On Wed, Mar 1, 2017 at 4:36 PM Michael Paquier wrote: > On Thu, Mar 2, 2017 at 5:53 AM, Zach Walton wrote: > > I was able to test 9.4.11 and am seeing the same behavior: > > > > postgres=# SELECT pg_is_in_recovery(), pg_last_xlog_receive_location(), > > pg_last_xlog_replay_location(); > > pg_is_in_recovery | pg_last_xlog_receive_location | > > pg_last_xlog_replay_location > > > ---+---+-- > > t | | 0/3000198 > > Okay, you said that you are using here streaming replication, but the > standby you are performing this query on seems just to be a hot > standby recovering WAL from a WAL archive, not via streaming. I would > bet that there is no WAL receiver running. > pg_last_xlog_receive_location() get the last WAL position received > from a streaming node, something that is set to NULL if there is no > streaming happening, while pg_last_xlog_replay_location() is set by > the startup process when replaying WAL records. > > Again I see no bugs here, you should check if a WAL receiver is > running on this standby server. > -- > Michael >
Re: [GENERAL] odd optimizer result, index condition "is not null" on column defined as "not null"
Hi, following up my own post: I noted that I included the "set enable_seqscan=off; ". But the results I mentioned are from before this statement. I also compared some more statements explain analyze verbose select min(id) from tbl_foo where created_at >= '2017-01-15' ; explain analyze verbose select id from tbl_foo where created_at >= '2017-01-15' order by id limit 1; Those 2 are the same, but the 2nd skips the "not null" index condition. explain analyze verbose select min(id) filter(where created_at >= '2017-01-15') from tbl_foo; explain analyze verbose select min(id) filter(where created_at >= '2017-01-15') from tbl_foo where created_at >= '2017-01-15'; They should also be considered the same, as the "where" only removes rows, that are skipped by the "filter" anyway. It seems the filter changes the plan to the other index. But adding the where reduces the amount of "rows" that is scanned on this index. This is nothing todo with the original question of the "is not null" condition on the "not null" field. But it seems that, if "created_at" is only in the where part, the optimizer does not consider using "created_at" from the index (and doing an index only scan). If "created_at" is in the select part, then the optimizer considers the "index only scan". (and even uses it for the "where" part) To check this I tried explain analyze verbose select min(created_at), min(id) filter(where created_at >= '2017-01-15') from tbl_foo; and it gives an index only as well. Out of interest, anyone with 9.6.2, does it yield the same results? On 03/03/2017 16:41, Martin F wrote: CREATE TABLE if not exists tbl_foo( id bigserial NOT NULL primary key, created_at timestamp without time zone NOT NULL ); create index tbl_foo_date on tbl_foo using btree (created_at,id); insert into tbl_foo (created_at) values ('2017-01-01'), ('2017-01-02'), ('2017-01-03'), ('2017-01-04'), ('2017-01-05'), ('2017-01-06'), ('2017-01-07'), ('2017-01-08'), ('2017-01-09'), ('2017-01-10'), ('2017-01-11'), ('2017-01-12'), ('2017-01-13'), ('2017-01-14'), ('2017-01-15'), ('2017-01-16'), ('2017-01-17'), ('2017-01-18'), ('2017-01-19'), ('2017-01-20'), ('2017-01-21'), ('2017-01-22'), ('2017-01-23'), ('2017-01-24'), ('2017-01-25'), ('2017-01-26'), ('2017-01-27'), ('2017-01-28'), ('2017-01-29'), ('2017-02-02'), ('2017-02-02'), ('2017-02-03'), ('2017-02-04'), ('2017-02-05'), ('2017-02-06'), ('2017-02-07'), ('2017-02-08'), ('2017-02-09'), ('2017-02-10'), ('2017-02-11'), ('2017-02-12'), ('2017-02-13'), ('2017-02-14'), ('2017-02-15'), ('2017-02-16'), ('2017-02-17'), ('2017-02-18'), ('2017-02-19'), ('2017-02-20'), ('2017-02-21'), ('2017-02-22'), ('2017-02-23'), ('2017-02-24'), ('2017-02-25'), ('2017-02-26'), ('2017-02-27'), ('2017-02-28'); analyze tbl_foo; explain analyze verbose select min(id) from tbl_foo where created_at >= '2017-01-15'; explain analyze verbose select min(id) filter(where created_at >= '2017-01-15') from tbl_foo; set enable_seqscan=off; explain analyze verbose select min(id) from tbl_foo where created_at >= '2017-01-15'; explain analyze verbose select min(id) filter(where created_at >= '2017-01-15') from tbl_foo; drop TABLE tbl_foo; -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] odd optimizer result, index condition "is not null" on column defined as "not null"
Martin F writes: >Index Cond: (tbl_foo.id IS NOT NULL) > only "id" is the pk, and declared "not null". > So why this index condition? The IS NOT NULL condition is generated as part of transforming a "min(x)" query into an indexscan, on the basis that "select min(x) from ..." is equivalent to "select x from ... where x is not null order by x limit 1". Without the NOT NULL check, that's an incorrect transformation. You're right that we could observe that the NOT NULL is implied by a table constraint and drop it, but it seems unlikely to be worth the planner cycles to do so; the condition doesn't cost much at runtime. (Also, plans that depend on table constraints for correctness have added bookkeeping costs from tracking such dependency.) > The select with filter choose an IMHO better plan >> Index Only Scan using tbl_foo_date on public.tbl_foo [ shrug... ] Can't get too excited about that. The first rule of working with the Postgres planner is that planning results on toy tables do not scale to large tables; too many of the cost factors are nonlinear. But the bigger picture here, which would become more obvious if you were working with a non-toy amount of data, is that you're asking the planner to choose between two bad options. Basically it can either scan the data in id order (using the pkey index) and stop when it hits the first row satisfying the created_at condition (which might be arbitrarily far in); or it can scan all the data satisfying the created_at condition (possibly using the other index to optimize that) and remember the smallest id seen while doing so. Your query with the aggregate FILTER condition is a further-pessimized version of the second approach, because it has to scan the *whole* index; a per-aggregate FILTER doesn't get applied as an index condition, but only while executing that aggregate. If you are concerned about the performance of this specific query shape, what you actually want is an index on (id, created_at). That allows stopping at the first index entry satisfying the created_at condition, knowing that it must have the min id value that does so. On a toy table the performance of any of these variants is going to be so close that it's not certain which one the planner will pick (and it will hardly matter anyway). On a large table the correctly-chosen index will make an enormous difference. regards, tom lane -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] odd optimizer result, index condition "is not null" on column defined as "not null"
Hi, following up my own post: I noted that I included the "set enable_seqscan=off; ". But the results I mentioned are from before this statement. I also compared some more statements explain analyze verbose select min(id) from tbl_foo where created_at >= '2017-01-15' ; explain analyze verbose select id from tbl_foo where created_at >= '2017-01-15' order by id limit 1; Those 2 are the same, but the 2nd skips the "not null" index condition. explain analyze verbose select min(id) filter(where created_at >= '2017-01-15') from tbl_foo; explain analyze verbose select min(id) filter(where created_at >= '2017-01-15') from tbl_foo where created_at >= '2017-01-15'; They should also be considered the same, as the "where" only removes rows, that are skipped by the "filter" anyway. It seems the filter changes the plan to the other index. But adding the where reduces the amount of "rows" that is scanned on this index. This is nothing todo with the original question of the "is not null" condition on the "not null" field. But it seems that, if "created_at" is only in the where part, the optimizer does not consider using "created_at" from the index (and doing an index only scan). If "created_at" is in the select part, then the optimizer considers the "index only scan". (and even uses it for the "where" part) To check this I tried explain analyze verbose select min(created_at), min(id) filter(where created_at >= '2017-01-15') from tbl_foo; and it gives an index only as well. Out of interest, anyone with 9.6.2, does it yield the same results? On 03/03/2017 16:41, Martin F wrote: CREATE TABLE if not exists tbl_foo( id bigserial NOT NULL primary key, created_at timestamp without time zone NOT NULL ); create index tbl_foo_date on tbl_foo using btree (created_at,id); insert into tbl_foo (created_at) values ('2017-01-01'), ('2017-01-02'), ('2017-01-03'), ('2017-01-04'), ('2017-01-05'), ('2017-01-06'), ('2017-01-07'), ('2017-01-08'), ('2017-01-09'), ('2017-01-10'), ('2017-01-11'), ('2017-01-12'), ('2017-01-13'), ('2017-01-14'), ('2017-01-15'), ('2017-01-16'), ('2017-01-17'), ('2017-01-18'), ('2017-01-19'), ('2017-01-20'), ('2017-01-21'), ('2017-01-22'), ('2017-01-23'), ('2017-01-24'), ('2017-01-25'), ('2017-01-26'), ('2017-01-27'), ('2017-01-28'), ('2017-01-29'), ('2017-02-02'), ('2017-02-02'), ('2017-02-03'), ('2017-02-04'), ('2017-02-05'), ('2017-02-06'), ('2017-02-07'), ('2017-02-08'), ('2017-02-09'), ('2017-02-10'), ('2017-02-11'), ('2017-02-12'), ('2017-02-13'), ('2017-02-14'), ('2017-02-15'), ('2017-02-16'), ('2017-02-17'), ('2017-02-18'), ('2017-02-19'), ('2017-02-20'), ('2017-02-21'), ('2017-02-22'), ('2017-02-23'), ('2017-02-24'), ('2017-02-25'), ('2017-02-26'), ('2017-02-27'), ('2017-02-28'); analyze tbl_foo; explain analyze verbose select min(id) from tbl_foo where created_at >= '2017-01-15'; explain analyze verbose select min(id) filter(where created_at >= '2017-01-15') from tbl_foo; set enable_seqscan=off; explain analyze verbose select min(id) from tbl_foo where created_at >= '2017-01-15'; explain analyze verbose select min(id) filter(where created_at >= '2017-01-15') from tbl_foo; drop TABLE tbl_foo; -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
[GENERAL] odd optimizer result, index condition "is not null" on column defined as "not null"
Hi. I am new, and not sure which mailinglist this should go to, so I start with the general list. (please advice, if I should send this to a more specific list) This is tested with postgresql 9.5.5 (Maybe someone can confirm, if it is the same with later versions, saving me the work to upgrade right now, thanks) See the sql below. The select without "filter" produces Result (cost=0.45..0.46 rows=1 width=0) (actual time=0.229..0.234 rows=1 loops=1) Output: $0 InitPlan 1 (returns $0) -> Limit (cost=0.14..0.45 rows=1 width=8) (actual time=0.161..0.166 rows=1 loops=1) Output: tbl_foo.id -> Index Scan using tbl_foo_pkey on public.tbl_foo (cost=0.14..13.28 rows=43 width=8) (actual time=0.045..0.045 rows=1 loops=1) Output: tbl_foo.id Index Cond: (tbl_foo.id IS NOT NULL) Filter: (tbl_foo.created_at >= '2017-01-15 00:00:00'::timestamp without time zone) Rows Removed by Filter: 14 Planning time: 1.792 ms Execution time: 0.273 ms Index Cond: (tbl_foo.id IS NOT NULL) only "id" is the pk, and declared "not null". So why this index condition? The select with filter choose an IMHO better plan Index Only Scan using tbl_foo_date on public.tbl_foo Should the first optimizer result be considered a bug? Should it be reported somewhere? CREATE TABLE if not exists tbl_foo( id bigserial NOT NULL primary key, created_at timestamp without time zone NOT NULL ); create index tbl_foo_date on tbl_foo using btree (created_at,id); insert into tbl_foo (created_at) values ('2017-01-01'), ('2017-01-02'), ('2017-01-03'), ('2017-01-04'), ('2017-01-05'), ('2017-01-06'), ('2017-01-07'), ('2017-01-08'), ('2017-01-09'), ('2017-01-10'), ('2017-01-11'), ('2017-01-12'), ('2017-01-13'), ('2017-01-14'), ('2017-01-15'), ('2017-01-16'), ('2017-01-17'), ('2017-01-18'), ('2017-01-19'), ('2017-01-20'), ('2017-01-21'), ('2017-01-22'), ('2017-01-23'), ('2017-01-24'), ('2017-01-25'), ('2017-01-26'), ('2017-01-27'), ('2017-01-28'), ('2017-01-29'), ('2017-02-02'), ('2017-02-02'), ('2017-02-03'), ('2017-02-04'), ('2017-02-05'), ('2017-02-06'), ('2017-02-07'), ('2017-02-08'), ('2017-02-09'), ('2017-02-10'), ('2017-02-11'), ('2017-02-12'), ('2017-02-13'), ('2017-02-14'), ('2017-02-15'), ('2017-02-16'), ('2017-02-17'), ('2017-02-18'), ('2017-02-19'), ('2017-02-20'), ('2017-02-21'), ('2017-02-22'), ('2017-02-23'), ('2017-02-24'), ('2017-02-25'), ('2017-02-26'), ('2017-02-27'), ('2017-02-28'); analyze tbl_foo; explain analyze verbose select min(id) from tbl_foo where created_at >= '2017-01-15'; explain analyze verbose select min(id) filter(where created_at >= '2017-01-15') from tbl_foo; set enable_seqscan=off; explain analyze verbose select min(id) from tbl_foo where created_at >= '2017-01-15'; explain analyze verbose select min(id) filter(where created_at >= '2017-01-15') from tbl_foo; drop TABLE tbl_foo; -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] PortalSuspended
Rui Pacheco writes: > Is there a way to force the backend to send a PortalSuspended message to > the front-end? In your Execute message, request fewer rows than you know the portal will produce. regards, tom lane -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Querying JSON Lists
On 03/02/2017 01:09 PM, Sven R. Kunze wrote: On 28.02.2017 17:33, Adrian Klaver wrote: On 02/26/2017 03:26 AM, Sven R. Kunze wrote: Hello everyone, playing around with jsonb and coming from this SO question http://stackoverflow.com/questions/19925641/check-if-a-postgres-json-array-contains-a-string I wonder why PostgreSQL behaves differently for text and integers on the ? and @> operators. Let's have a look at 4 different but similar queries: -- A) ? + text select '{"food": ["12","34","45"]}'::jsonb->'food' ? '12'; ?column? -- t -- B) ? + integer select '{"food": [12,34,45]}'::jsonb->'food' ? 12; ERROR: operator does not exist: jsonb ? integer LINE 1: select '{"food": [12,34,45]}'::jsonb->'food' ? 12; ^ HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts. https://www.postgresql.org/docs/9.6/static/datatype-json.html#JSON-CONTAINMENT "jsonb also has an existence operator, which is a variation on the theme of containment: it tests whether a string (given as a text value) appears as an object key or array element at the top level of the jsonb value. These examples return true except as noted -- String exists as array element: SELECT '["foo", "bar", "baz"]'::jsonb ? 'bar'; " -- C) @> + text select '{"food": ["12","34","45"]}'::jsonb->'food' @> '["12"]', '{"food": ["12","34","45"]}'::jsonb->'food' @> '"12"', '{"food": ["12","34","45"]}'::jsonb->'food' @> '12'; ?column? | ?column? | ?column? --+--+-- t| t| f -- D) @> + integer select '{"food": [12,34,45]}'::jsonb->'food' @> '[12]', '{"food": [12,34,45]}'::jsonb->'food' @> '12';--, '{"food": [12,34,45]}'::jsonb->'food' @> 12; ?column? | ?column? --+-- t| t Now my questions: 1) Why does A) work? Docs tells us that ? works for keys, not values. 2) Why does B) not work although A) works? 3) Why do the variants without the brackets on the right side of @> work in C) and D)? Is there json data where their results differ from the ones with the brackets? 4) What is the recommended way of testing inclusion in json lists? I have not worked through your examples, but I suspect the answer's lie here: https://www.postgresql.org/docs/9.6/static/datatype-json.html#JSON-CONTAINMENT 8.14.3. jsonb Containment and Existence More details yes, but not really an explanation on the 'why'. Especially not on 2) https://www.postgresql.org/docs/9.6/static/datatype-json.html#JSON-CONTAINMENT "-- Object containing pairs of keys and values -- Note that object keys must always be quoted strings SELECT '{"bar": "baz", "balance": 7.77, "active": false}'::json;" Which I believe comes from: https://tools.ietf.org/html/rfc7159 "4. Objects An object structure is represented as a pair of curly brackets surrounding zero or more name/value pairs (or members). A name is a string. A single colon comes after each name, separating the name from the value. A single comma separates a value from a following name. The names within an object SHOULD be unique. " https://www.postgresql.org/docs/9.6/static/functions-json.html As to why it works on JSON arrays: Table 9-43. Additional jsonb Operators " ? textDoes the string exist as a top-level key within the JSON value? " So to be picky it not does call out JSON object it says JSON value. And right above the table: " For a full description of jsonb containment and existence semantics, see Section 8.14.3. Section 8.14.4 describes how these operators can be used to effectively index jsonb data." As to how that behavior was decided on I have no idea, it just is. and 3). These feel like holes in the implementation. As to this: test=> select '["12","34","45"]'::jsonb @> '"12"'; ?column? -- t " As a special exception to the general principle that the structures must match, an array may contain a primitive value: -- This array contains the primitive string value: SELECT '["foo", "bar"]'::jsonb @> '"bar"'::jsonb; -- This exception is not reciprocal -- non-containment is reported here: SELECT '"bar"'::jsonb @> '["bar"]'::jsonb; -- yields false " Though there looks to be some implicit casting going on: test=> select '["12","34","45"]'::jsonb @> '"12"'::text; ERROR: operator does not exist: jsonb @> text LINE 1: select '["12","34","45"]'::jsonb @> '"12"'::text; to get '"12"' to be '"12"'::jsonb. As to why, I don't know. Sven -- Adrian Klaver adrian.kla...@aklaver.com -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Autoanalyze oddity
On 03/03/2017 12:33 AM, Peter J. Holzer wrote: This is with PostgreSQL 9.5.6 on Debian Linux. I noticed that according to pg_stat_user_tables autoanalyze has never run on a lot of tables. Here is one example: wdsah=> select * from pg_stat_user_tables where schemaname='public' and relname='facttable_wds_indexstats'; ─[ RECORD 1 ]───┬─ relid │ 112723 schemaname │ public relname │ facttable_wds_indexstats seq_scan│ 569 seq_tup_read│ 474779212 idx_scan│ 59184 idx_tup_fetch │ 59184 n_tup_ins │ 47128 n_tup_upd │ 0 n_tup_del │ 0 n_tup_hot_upd │ 0 n_live_tup │ 47128 n_dead_tup │ 0 n_mod_since_analyze │ 47128 last_vacuum │ (∅) last_autovacuum │ (∅) last_analyze│ (∅) last_autoanalyze│ (∅) vacuum_count│ 0 autovacuum_count│ 0 analyze_count │ 0 autoanalyze_count │ 0 wdsah=> select count(*) from facttable_wds_indexstats; count 857992 (1 row) So, n_live_tup is way off, and n_tup_ins and n_mod_since_analyze also seem to be wrong. Looks like this hasn't been updated in a year or so. But track_counts is on: wdsah=> show track_counts; track_counts ── on (1 row) What are your settings for autovacuum?: https://www.postgresql.org/docs/9.5/static/runtime-config-autovacuum.html Have the storage parameters for the table been altered?: https://www.postgresql.org/docs/9.5/static/sql-createtable.html#SQL-CREATETABLE-STORAGE-PARAMETERS And even if it wasn't, shouldn't the autovacuum daemon notice that n_mod_since_analyze is greater than n_live_tup * autovacuum_analyze_scale_factor and run an autoanalyze? That value is added to autovacuum_analyze_threshold: autovacuum_analyze_scale_factor (floating point) Specifies a fraction of the table size to add to autovacuum_analyze_threshold when deciding whether to trigger an ANALYZE. The default is 0.1 (10% of table size). This parameter can only be set in the postgresql.conf file or on the server command line; but the setting can be overridden for individual tables by changing table storage parameters. But the really weird thing is that pg_stats seems to be reasonably current: I see entries in most_common_vals which were only inserted in January. Is it possible that autoanalyze runs without updating pg_stat_user_tables? hp -- Adrian Klaver adrian.kla...@aklaver.com -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Full Text Search combined with Fuzzy
On 03.03.2017 16:17, Nicolas Paris wrote: Nice ! I do have 9.6 version. Would this kind of index could handle more than 20M large texts ? The recheck condition looks ressource consuming. You are right. I think pg_trgm will be not good for such large texts, unfortunately. The full text index + phrase search + synonym dictionnary is the only other alternativ to deal with typo-phrase mining ? I suppose there are no other options now. Though, prefix search maybe will help you [1]. Is there any possibility in the future to add typo in the full text road-map ? As far as I know, there is no plans in the near future to add similarity full text search. 1. https://www.postgresql.org/docs/current/static/textsearch-dictionaries.html -- Artur Zakirov Postgres Professional: http://www.postgrespro.com Russian Postgres Company -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] DISTINCT vs GROUP BY - was Re: is (not) distinct from
On 3 March 2017 at 18:26, George Neuner wrote: > I know most people here don't pay much - or any - attention to > SQLServer, however there was an interesting article recently regarding > significant performance differences between DISTINCT and GROUP BY as > used to remove duplicates. > > https://sqlperformance.com/2017/01/t-sql-queries/surprises-assumptions-group-by-distinct > > > Now I'm wondering if something similar might be lurking in Postgresql? Yes things lurk there in PostgreSQL too. But to be honest I find the examples in the URL you included a bit strange. There's almost certainly going to be a table called "orders" that you'd use for the outer part of the query. In that case the orderid would already be unique. To do the same in PostgreSQL you'd just use: select orderid, string_agg(description,'|') from orderitems group by orderid; assuming all orders had at least one line, you'd get the same result. In more general terms, PostgreSQL will allow you to GROUP BY and non-aggregated columns which are functionally dependent on the GROUP BY clause, for example: SELECT parts.partcode,parts.description,sum(sales.quantity) from sales inner join parts on sales.partcode = parts.partcode GROUP BY parts.partcode; Assuming that parts.partcode is the PRIMARY KEY of parts, this query is legal in PostgreSQL. In some other databases, and I believe SQL Server might be one of them, you would have been forced to include part.description in the GROUP BY clause. Since PostgreSQL 9.6, if you'd have done the same with that, internally the database would ignore the parts.description in the GROUP BY clause, as its smart enough to know that including parts.description in the clause is not going to change anything as the description is always the same for each parts.partcode, and no two records can share the same partcode. There's no such optimisation when it comes to DISTINCT. In PostgreSQL as of today DISTINCT is a bit naive, and will just uniquify the results on each column in the select clause. Although quite possibly the same optimisation could apply to DISTINCT too, just nobody has thought to add it yet. In short, the main difference is going to be the fewer columns you're using to identify the groups the better. If you included all of the columns in the GROUP BY clause as you put in the select list with the DISTINCT query then in most cases the performance would be the same. I believe the only exception to this is in regards to parallel query, as currently only GROUP BYs may be parallelised, not DISTINCT. Historically with older now unsupported versions of PostgreSQL (pre 8.4) you may have also preferred to use GROUP BY over DISTINCT as GROUP BY could be implemented internally by sorting or hashing the results, whereas DISTINCT used to only be implemented by Sorting the results. Although this has long since been the case. -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Full Text Search combined with Fuzzy
Le 03 mars 2017 à 14:08, Artur Zakirov écrivait : > On 03.03.2017 15:49, Nicolas Paris wrote: > > > >Hi Oleg, > > > >Thanks. I thought pgtrgm was not able to index my long texts because of > >limitation of 8191 bytes per index row for btree. > > > >Then I found out it is possible to use pgtrgm over a GIN/GIST index. > >My final use case is phrase mining in texts. > > > >I want my application returns texts that contains approximatly the user > >entry: > > > >Eg: user search "Hello Word" > >a text containing "blah blah blah hello world blah blah blah" would be > >returned. > > > >Test: > >postgres=# CREATE table test_trgm (texts text); > >CREATE TABLE > >postgres=# CREATE INDEX ON test_trgm USING GIN(texts gin_trgm_ops); > >CREATE INDEX > >postgres=# SET enable_seqscan = OFF; > >SET > >postgres=# insert into test_trgm VALUES ('blah blah blah hello world blah > >blah blah'); > >INSERT 0 1 > >postgres=# insert into test_trgm VALUES ('blah blah blah hello word blah > >blah blah'); > >INSERT 0 1 > >postgres=# SELECT texts, similarity(texts, 'hello word') FROM test_trgm > >WHERE texts % 'hello word'; > > texts | similarity > >---+ > > blah blah blah hello world blah blah blah | 0.473684 > > blah blah blah hello word blah blah blah | 0.6875 > >(2 rows) > > > >postgres=# EXPLAIN SELECT texts, similarity(texts, 'hello word') FROM > >test_trgm WHERE texts % 'hello word'; > >QUERY PLAN > >--- > > Bitmap Heap Scan on test_trgm (cost=52.01..56.03 rows=1 width=32) > > Recheck Cond: (texts % 'hello word'::text) > > -> Bitmap Index Scan on test_trgm_texts_idx (cost=0.00..52.01 rows=1 > > width=0) > > Index Cond: (texts % 'hello word'::text) > >(4 rows) > > > >Conclusion: If I d'say 0.4 is my threshold, would this methodology meet > >my requirements ? > > > >Thanks for the help ! > > > > Hello, > > If you use PostgreSQL 9.6, then word_similarity() can help you [1]. For > example: > > postgres=# SELECT texts, word_similarity('hello word', texts) FROM test_trgm > WHERE 'hello word' <% texts; >texts | word_similarity > ---+- > blah blah blah hello world blah blah blah |0.818182 > blah blah blah hello word blah blah blah | 1 > (2 rows) > > 1. https://www.postgresql.org/docs/9.6/static/pgtrgm.html > Nice ! I do have 9.6 version. Would this kind of index could handle more than 20M large texts ? The recheck condition looks ressource consuming. The full text index + phrase search + synonym dictionnary is the only other alternativ to deal with typo-phrase mining ? Is there any possibility in the future to add typo in the full text road-map ? Thanks, > -- > Artur Zakirov > Postgres Professional: http://www.postgrespro.com > Russian Postgres Company -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Full Text Search combined with Fuzzy
On 03.03.2017 15:49, Nicolas Paris wrote: Hi Oleg, Thanks. I thought pgtrgm was not able to index my long texts because of limitation of 8191 bytes per index row for btree. Then I found out it is possible to use pgtrgm over a GIN/GIST index. My final use case is phrase mining in texts. I want my application returns texts that contains approximatly the user entry: Eg: user search "Hello Word" a text containing "blah blah blah hello world blah blah blah" would be returned. Test: postgres=# CREATE table test_trgm (texts text); CREATE TABLE postgres=# CREATE INDEX ON test_trgm USING GIN(texts gin_trgm_ops); CREATE INDEX postgres=# SET enable_seqscan = OFF; SET postgres=# insert into test_trgm VALUES ('blah blah blah hello world blah blah blah'); INSERT 0 1 postgres=# insert into test_trgm VALUES ('blah blah blah hello word blah blah blah'); INSERT 0 1 postgres=# SELECT texts, similarity(texts, 'hello word') FROM test_trgm WHERE texts % 'hello word'; texts | similarity ---+ blah blah blah hello world blah blah blah | 0.473684 blah blah blah hello word blah blah blah | 0.6875 (2 rows) postgres=# EXPLAIN SELECT texts, similarity(texts, 'hello word') FROM test_trgm WHERE texts % 'hello word'; QUERY PLAN --- Bitmap Heap Scan on test_trgm (cost=52.01..56.03 rows=1 width=32) Recheck Cond: (texts % 'hello word'::text) -> Bitmap Index Scan on test_trgm_texts_idx (cost=0.00..52.01 rows=1 width=0) Index Cond: (texts % 'hello word'::text) (4 rows) Conclusion: If I d'say 0.4 is my threshold, would this methodology meet my requirements ? Thanks for the help ! Hello, If you use PostgreSQL 9.6, then word_similarity() can help you [1]. For example: postgres=# SELECT texts, word_similarity('hello word', texts) FROM test_trgm WHERE 'hello word' <% texts; texts | word_similarity ---+- blah blah blah hello world blah blah blah |0.818182 blah blah blah hello word blah blah blah | 1 (2 rows) 1. https://www.postgresql.org/docs/9.6/static/pgtrgm.html -- Artur Zakirov Postgres Professional: http://www.postgrespro.com Russian Postgres Company -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] ERROR: functions in index expression must be marked IMMUTABLE
On 3 March 2017 at 12:17, Sven R. Kunze wrote: > On 03.03.2017 11:43, Geoff Winkless wrote: > > One alternative would be to make to_date accept all language variants of > months simultaneously. A quick search of google suggests that there aren't > any overlaps between languages (ie where one language uses "Foo" for March > and another uses "Foo" for May), although you would have to do some more > intense research to be sure. As far as I can see there's no other reason > why to_date would need to be marked as stable/volatile, is there? > > > it seems there are overlapping short months: > Trust the Balkan states to find something to disagree over :) Oh well, that scrubs that idea then. Geoff
Re: [GENERAL] Full Text Search combined with Fuzzy
Le 27 févr. 2017 à 10:32, Oleg Bartunov écrivait : > > > On Sun, Feb 26, 2017 at 3:52 PM, Nicolas Paris wrote: > > Hello, > > AFAIK there is no built-in way to combine full text search and fuzzy > matching > (https://www.postgresql.org/docs/current/static/fuzzystrmatch.html). > By example, phrase searching with tipos in it. > > First I don't know if postgresql concurrents (lucene based...) are able > to do so. > > > Usually, https://www.postgresql.org/docs/current/static/pgtrgm.html is used > for > this. > Hi Oleg, Thanks. I thought pgtrgm was not able to index my long texts because of limitation of 8191 bytes per index row for btree. Then I found out it is possible to use pgtrgm over a GIN/GIST index. My final use case is phrase mining in texts. I want my application returns texts that contains approximatly the user entry: Eg: user search "Hello Word" a text containing "blah blah blah hello world blah blah blah" would be returned. Test: postgres=# CREATE table test_trgm (texts text); CREATE TABLE postgres=# CREATE INDEX ON test_trgm USING GIN(texts gin_trgm_ops); CREATE INDEX postgres=# SET enable_seqscan = OFF; SET postgres=# insert into test_trgm VALUES ('blah blah blah hello world blah blah blah'); INSERT 0 1 postgres=# insert into test_trgm VALUES ('blah blah blah hello word blah blah blah'); INSERT 0 1 postgres=# SELECT texts, similarity(texts, 'hello word') FROM test_trgm WHERE texts % 'hello word'; texts | similarity ---+ blah blah blah hello world blah blah blah | 0.473684 blah blah blah hello word blah blah blah | 0.6875 (2 rows) postgres=# EXPLAIN SELECT texts, similarity(texts, 'hello word') FROM test_trgm WHERE texts % 'hello word'; QUERY PLAN --- Bitmap Heap Scan on test_trgm (cost=52.01..56.03 rows=1 width=32) Recheck Cond: (texts % 'hello word'::text) -> Bitmap Index Scan on test_trgm_texts_idx (cost=0.00..52.01 rows=1 width=0) Index Cond: (texts % 'hello word'::text) (4 rows) Conclusion: If I d'say 0.4 is my threshold, would this methodology meet my requirements ? Thanks for the help ! > > Second, is such feature is in the road map ? > > Third, I wonder if it is a good idea to use the postgresql synonyms > feature for such prupose.(https://www.postgresql.org/docs/current/static/ > textsearch-dictionaries.html) > I mean, building up a synonyms dictionnary containing tipos. By eg: > > postgres pgsql > postgresql pgsql > postgrez pgsql > postgre pgsql > gogle googl > gooogle googl > > There is multiple way to build such dictionary. But my question is about > the implementation of dictionnaries in postgresql: Is postgresql > supposed to take advantage of billion entries dictionaries ? > > > dictionary is just a program, so it's up to developer how to write efficient > program to deal with billion entries. Specifically to synonym dictionary, it's > not intended to work with a lot of entries. btw, have a look on contrib/ > dict_xsyn dictionary, which is more flexible than synonym. > > > Thanks by advance for you answers, > > > -- > Sent via pgsql-general mailing list (pgsql-general@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-general > > -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] ERROR: functions in index expression must be marked IMMUTABLE
On 03.03.2017 11:43, Geoff Winkless wrote: One alternative would be to make to_date accept all language variants of months simultaneously. A quick search of google suggests that there aren't any overlaps between languages (ie where one language uses "Foo" for March and another uses "Foo" for May), although you would have to do some more intense research to be sure. As far as I can see there's no other reason why to_date would need to be marked as stable/volatile, is there? As a side-note, it seems there are overlapping short months: Using this webpage http://web.library.yale.edu/cataloging/months.htm this JS(Firefox) JSON.stringify([for (x of document.querySelectorAll('table.grid tr td')) x.textContent]) this Python import pprint from collections import defaultdict def chunks(l, n): """Yield successive n-sized chunks from l.""" for i in range(0, len(l), n): yield l[i:i + n] a= a=list(chunks(a, 13)) dd=defaultdict(list) for l in a: for i, m in enumerate(l): if i == 0: continue dd[m].append((i, l[0])) pprint.pprint(dict(dd)) {'\nmart\n': [(3, 'Serbian')], 'Ag.': [(8, 'Indonesian')], 'Agustos': [(8, 'Turkish')], 'Apr': [(4, 'Latin')], 'Apr.': [(4, 'English'), (4, 'German'), (4, 'Greek, Modern'), (4, 'Indonesian'), (4, 'Malaysian'), (4, 'Romanian')], 'Aralik': [(12, 'Turkish')], 'Aug.': [(8, 'English'), (8, 'German'), (8, 'Greek, Modern'), (8, 'Latin'), (8, 'Romanian')], 'Awst': [(8, 'Welsh')], 'Chwef.': [(2, 'Welsh')], 'Dec.': [(12, 'English'), (12, 'Latin'), (12, 'Romanian')], 'Dek.': [(12, 'Greek, Modern')], 'Des.': [(12, 'Indonesian')], 'Dez.': [(12, 'German')], 'Dis.': [(12, 'Malaysian')], 'Ebr.': [(4, 'Welsh')], 'Ekim': [(10, 'Turkish')], 'Eylul': [(9, 'Turkish')], 'Feb.': [(2, 'English'), (2, 'German'), (2, 'Malaysian'), (2, 'Romanian')], 'Febr.': [(2, 'Latin')], 'Gorff.': [(7, 'Welsh')], 'Haziran': [(6, 'Turkish')], 'Hyd.': [(10, 'Welsh')], 'Ian.': [(1, 'Greek, Modern'), (1, 'Latin'), (1, 'Romanian')], 'Ion.': [(1, 'Welsh')], 'Ioul.': [(7, 'Greek, Modern')], 'Ioun.': [(6, 'Greek, Modern')], 'Iul.': [(7, 'Latin')], 'Iulie': [(7, 'Romanian')], 'Iun.': [(6, 'Latin')], 'Iunie': [(6, 'Romanian')], 'Jan.': [(1, 'English'), (1, 'Malaysian')], 'Jan./Djan.\xc2\xa0': [(1, 'Indonesian')], 'Jan./J\xc3\xa4n.': [(1, 'German')], 'Julai': [(7, 'Malaysian')], 'Juli': [(7, 'German')], 'Juli/Djuli': [(7, 'Indonesian')], 'July': [(7, 'English')], 'Jun': [(6, 'Malaysian')], 'June': [(6, 'English')], 'Juni': [(6, 'German')], 'Juni/Djuni': [(6, 'Indonesian')], 'Kasim': [(11, 'Turkish')], 'Mac': [(3, 'Malaysian')], 'Mai': [(5, 'German'), (5, 'Romanian'), (5, 'Welsh')], 'Mai.': [(5, 'Latin')], 'Maios': [(5, 'Greek, Modern')], 'Mar.': [(3, 'English'), (3, 'Romanian')], 'Mart.': [(3, 'Greek, Modern'), (3, 'Latin')], 'Maw.': [(3, 'Welsh')], 'May': [(5, 'English')], 'Mayis': [(5, 'Turkish')], 'Medi': [(9, 'Welsh')], 'Meh.': [(6, 'Welsh')], 'Mei': [(5, 'Malaysian')], 'Mei/Mai': [(5, 'Indonesian')], 'Mrt.': [(3, 'Indonesian')], 'M\xc3\xa4rz': [(3, 'German')], 'Nisan': [(4, 'Turkish')], 'Noem.': [(11, 'Greek, Modern')], 'Noiem.': [(11, 'Romanian')], 'Nop.': [(11, 'Indonesian')], 'Nov.': [(11, 'English'), (11, 'German'), (11, 'Latin'), (11, 'Malaysian')], 'Ocak': [(1, 'Turkish')], 'Oct.': [(10, 'English'), (10, 'Latin'), (10, 'Romanian')], 'Og': [(8, 'Malaysian')], 'Okt.': [(10, 'German'), (10, 'Greek, Modern'), (10, 'Indonesian'), (10, 'Malaysian')], 'Peb.': [(2, 'Indonesian')], 'Phevr.': [(2, 'Greek, Modern')], 'Rhag.': [(12, 'Welsh')], 'Saus.': [(1, 'Lithuanian')], 'Sept.': [(9, 'English'), (9, 'German'), (9, 'Greek, Modern'), (9, 'Indonesian'), (9, 'Latin'), (9, 'Malaysian'), (9, 'Romanian')], 'Subat': [(2, 'Turkish')], 'Tach.': [(11, 'Welsh')], 'Temmuz': [(7, 'Turkish')], 'abr.': [(4, 'Spanish')], 'abril': [(4, 'Portuguese')], 'ag.': [(8, 'Italian')], 'agosto': [(8, 'Portuguese'), (8, 'Spanish')], 'ao\xc3\xbbt': [(8, 'French')], 'apr.': [(4, 'Dutch'), (4, 'Estonian'), (4, 'Italian'), (4, 'Latvian'), (4, 'Russian')], 'apr./mali traven': [(4, 'Slovenian')], 'april': [(4, 'Bosnian'), (4, 'Bulgarian'), (4, 'Danish'), (4, 'Norwegian'), (4, 'Serbian'), (4, 'Swedish')], 'aug.': [(8, 'Bosnian'), (8, 'Danish'), (8, 'Dutch'), (8, 'Estonian'), (8, 'Hungarian'), (8, 'Latvian'), (8, 'Norwegian'), (8, 'Swedish')], 'avg.': [(8, 'Bulgarian'), (8, 'Russian'), (8, 'Serbian')], 'avg./veliki\xc2\xa0srpan': [(8, 'Slovenian')], 'avril': [(4, 'French')], 'bal.': [(4, 'Lithuanian')], 'ber.': [(3, 'Ukranian')], 'birz': [(6, 'Lithuanian')], 'brez.':
[GENERAL] PortalSuspended
Hello, Is there a way to force the backend to send a PortalSuspended message to the front-end? Perhaps emulate a particular load or a specific sequence of sql commands sent from the front-end? -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] ERROR: functions in index expression must be marked IMMUTABLE
On 03.03.2017 11:43, Geoff Winkless wrote: One alternative would be to make to_date accept all language variants of months simultaneously. A quick search of google suggests that there aren't any overlaps between languages (ie where one language uses "Foo" for March and another uses "Foo" for May), although you would have to do some more intense research to be sure. As far as I can see there's no other reason why to_date would need to be marked as stable/volatile, is there? I don't think so. It could be viable. On the down side I imagine it would involve some potentially-prohibitively-large lookup tables; it would also end up with a technical incompatibility in that what ANSI SQL would reject as not-a-date might be parsed as a date. There is another issue: languages change (admittedly very slowly) but I wouldn't want PostgreSQL to be incompatible with future generations. Your performance argument weighs heavier, though. I'm not in a position to judge if either of those would be acceptable. Do you think I should post to pgsql-hackers? Sven
Re: [GENERAL] ERROR: functions in index expression must be marked IMMUTABLE
On 1 March 2017 at 14:23, Sven R. Kunze wrote: > I don't consider rolling an UDF the best alternative especially after > having looked through many solution proposals on the Web which just take an > mutable expression and wrap them up in an immutable function. > One alternative would be to make to_date accept all language variants of months simultaneously. A quick search of google suggests that there aren't any overlaps between languages (ie where one language uses "Foo" for March and another uses "Foo" for May), although you would have to do some more intense research to be sure. As far as I can see there's no other reason why to_date would need to be marked as stable/volatile, is there? On the down side I imagine it would involve some potentially-prohibitively-large lookup tables; it would also end up with a technical incompatibility in that what ANSI SQL would reject as not-a-date might be parsed as a date. I'm not in a position to judge if either of those would be acceptable. Geoff
Re: [GENERAL] DISTINCT vs GROUP BY - was Re: is (not) distinct from
On 03.03.2017 06:26, George Neuner wrote: I know most people here don't pay much - or any - attention to SQLServer, however there was an interesting article recently regarding significant performance differences between DISTINCT and GROUP BY as used to remove duplicates. https://sqlperformance.com/2017/01/t-sql-queries/surprises-assumptions-group-by-distinct On a similar note, this is also an interesting read about the topic of distinct vs group by: https://blogs.oracle.com/developer/entry/counting_with_oracle_is_faster Interesting is the performance difference between integers and strings for PostgreSQL which doesn't exist for Oracle. I also tried rewriting "select distinct" to "select group by" using PostgreSQL. It didn't help; it was even worse (see appendix). I'll get around to doing some testing soon. For now, I am just asking if anyone has ever run into something like this? Yes, my team did. We use Django on a daily basis to generate SQL queries. In case of model-spanning queries, a lot of joining and duplications are involved. Distinct is the "generally" accepted way to remedy the situation but it's actually more like Tom said: distinct is a band-aid here. UNIONS and SUBSELECTs would be better I guess. Sven ** Appendix ** # \d docs Table "public.docs" Column | Type | Modifiers +-+--- id | integer | not null default nextval('docs_id_seq'::regclass) meta | jsonb | Indexes: "docs_pkey" PRIMARY KEY, btree (id) # explain analyze select count(distinct meta->>'blood_group') from docs; QUERY PLAN Aggregate (cost=760497.00..760497.01 rows=1 width=449) (actual time=37631.727..37631.727 rows=1 loops=1) -> Seq Scan on docs (cost=0.00..710497.00 rows=1000 width=449) (actual time=0.500..3999.417 rows=1000 loops=1) Planning time: 0.211 ms Execution time: 37631.829 ms (4 rows) # explain analyze select count(*) from (select meta->>'blood_group' from docs group by meta->>'blood_group') as x; QUERY PLAN Aggregate (cost=4441923.83..4441923.84 rows=1 width=0) (actual time=41189.472..41189.472 rows=1 loops=1) -> Group (cost=4241923.83..4316923.83 rows=1000 width=449) (actual time=31303.690..41189.455 rows=8 loops=1) Group Key: ((docs.meta ->> 'blood_group'::text)) -> Sort (cost=4241923.83..4266923.83 rows=1000 width=449) (actual time=31303.686..40475.227 rows=1000 loops=1) Sort Key: ((docs.meta ->> 'blood_group'::text)) Sort Method: external merge Disk: 129328kB -> Seq Scan on docs (cost=0.00..735497.00 rows=1000 width=449) (actual time=0.349..6433.691 rows=1000 loops=1) Planning time: 2.189 ms Execution time: 41203.669 ms (9 rows)
Re: [GENERAL] PG on SSD
On 03/03/2017 01:31, Scott Marlowe wrote: On Thu, Mar 2, 2017 at 12:42 PM, scott ribe wrote: Is it reasonable to run PG on a mirrored pair of something like the Intel SSD DC 3610 series? (For example: http://ark.intel.com/products/82935/Intel-SSD-DC-S3610-Series-480GB-2_5in-SATA-6Gbs-20nm-MLC) I'd *hope* that anything Intel classifies as a "Data Center SSD" would be reasonably reliable, have actually-working power loss protection etc, but is that the case? From the spec sheet they certainly seem to be safe against power loss. I'd still test by pulling the power cables while running benchmarks to be sure. I've used the other Intel enterprise class ssds with good results on the power plug pull tests. + Intel not only markets this as "Data Center SSD", moreover this seems to be in the respective high-end range within "Data Center SSDs". -- Achilleas Mantzios IT DEV Lead IT DEPT Dynacom Tankers Mgmt -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Querying JSON Lists
On 28.02.2017 17:33, Adrian Klaver wrote: On 02/26/2017 03:26 AM, Sven R. Kunze wrote: Hello everyone, playing around with jsonb and coming from this SO question http://stackoverflow.com/questions/19925641/check-if-a-postgres-json-array-contains-a-string I wonder why PostgreSQL behaves differently for text and integers on the ? and @> operators. Let's have a look at 4 different but similar queries: -- A) ? + text select '{"food": ["12","34","45"]}'::jsonb->'food' ? '12'; ?column? -- t -- B) ? + integer select '{"food": [12,34,45]}'::jsonb->'food' ? 12; ERROR: operator does not exist: jsonb ? integer LINE 1: select '{"food": [12,34,45]}'::jsonb->'food' ? 12; ^ HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts. https://www.postgresql.org/docs/9.6/static/datatype-json.html#JSON-CONTAINMENT "jsonb also has an existence operator, which is a variation on the theme of containment: it tests whether a string (given as a text value) appears as an object key or array element at the top level of the jsonb value. These examples return true except as noted -- String exists as array element: SELECT '["foo", "bar", "baz"]'::jsonb ? 'bar'; " -- C) @> + text select '{"food": ["12","34","45"]}'::jsonb->'food' @> '["12"]', '{"food": ["12","34","45"]}'::jsonb->'food' @> '"12"', '{"food": ["12","34","45"]}'::jsonb->'food' @> '12'; ?column? | ?column? | ?column? --+--+-- t| t| f -- D) @> + integer select '{"food": [12,34,45]}'::jsonb->'food' @> '[12]', '{"food": [12,34,45]}'::jsonb->'food' @> '12';--, '{"food": [12,34,45]}'::jsonb->'food' @> 12; ?column? | ?column? --+-- t| t Now my questions: 1) Why does A) work? Docs tells us that ? works for keys, not values. 2) Why does B) not work although A) works? 3) Why do the variants without the brackets on the right side of @> work in C) and D)? Is there json data where their results differ from the ones with the brackets? 4) What is the recommended way of testing inclusion in json lists? I have not worked through your examples, but I suspect the answer's lie here: https://www.postgresql.org/docs/9.6/static/datatype-json.html#JSON-CONTAINMENT 8.14.3. jsonb Containment and Existence More details yes, but not really an explanation on the 'why'. Especially not on 2) and 3). These feel like holes in the implementation. Sven -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
[GENERAL] Autoanalyze oddity
This is with PostgreSQL 9.5.6 on Debian Linux. I noticed that according to pg_stat_user_tables autoanalyze has never run on a lot of tables. Here is one example: wdsah=> select * from pg_stat_user_tables where schemaname='public' and relname='facttable_wds_indexstats'; ─[ RECORD 1 ]───┬─ relid │ 112723 schemaname │ public relname │ facttable_wds_indexstats seq_scan│ 569 seq_tup_read│ 474779212 idx_scan│ 59184 idx_tup_fetch │ 59184 n_tup_ins │ 47128 n_tup_upd │ 0 n_tup_del │ 0 n_tup_hot_upd │ 0 n_live_tup │ 47128 n_dead_tup │ 0 n_mod_since_analyze │ 47128 last_vacuum │ (∅) last_autovacuum │ (∅) last_analyze│ (∅) last_autoanalyze│ (∅) vacuum_count│ 0 autovacuum_count│ 0 analyze_count │ 0 autoanalyze_count │ 0 wdsah=> select count(*) from facttable_wds_indexstats; count 857992 (1 row) So, n_live_tup is way off, and n_tup_ins and n_mod_since_analyze also seem to be wrong. Looks like this hasn't been updated in a year or so. But track_counts is on: wdsah=> show track_counts; track_counts ── on (1 row) And even if it wasn't, shouldn't the autovacuum daemon notice that n_mod_since_analyze is greater than n_live_tup * autovacuum_analyze_scale_factor and run an autoanalyze? But the really weird thing is that pg_stats seems to be reasonably current: I see entries in most_common_vals which were only inserted in January. Is it possible that autoanalyze runs without updating pg_stat_user_tables? hp -- _ | Peter J. Holzer| A coding theorist is someone who doesn't |_|_) || think Alice is crazy. | | | h...@hjp.at | -- John Gordon __/ | http://www.hjp.at/ |http://downlode.org/Etext/alicebob.html signature.asc Description: Digital signature