Re: [PERFORM] bad estimates
I haven't come up with any great ideas for this one. It might be interesting to compare the explain analyze output from the distinct on query with and without seqscans enabled. ---(end of broadcast)--- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [PERFORM] bad estimates
Bruno Wolff III wrote: I haven't come up with any great ideas for this one. It might be interesting to compare the explain analyze output from the distinct on query with and without seqscans enabled. Can't do that comparison. Remember, with seqscan it fails. (Oh, and that nested loops solution I thought was fast actually took 31 minutes versus 29 for index scan in 7.4b2.) I ran another query across the same data: select price_date, count(*) from day_ends group by price_date; It used a table scan and hashed aggregates, and it ran in 5.5 minutes. Considering that, pgsql should be able to do the query that I had been running in a little more time than that. So... From what I've learned, we want to convince the optimizer to use a table scan; that's a good thing. I want it to use hashed aggregates, but I can't convince it to (unless maybe I removed all of the statistics.) To use group aggregates, it first sorts the results of the table scan (all 17 million rows!) There ought to be some way to tell pgsql not to do sorts above a certain size. In this case, if I set enable_sort=false, it goes back to the index scan. If I then set enable_indexscan=false, it goes back to sorting. ---(end of broadcast)--- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PERFORM] bad estimates
Bruno Wolff III wrote: I haven't come up with any great ideas for this one. It might be interesting to compare the explain analyze output from the distinct on query with and without seqscans enabled. After digging through planner code, I found that bumping up the sort_mem will make the planner prefer a full table scan and hashed aggregation. The sort memory is where the hash table is stored. In the end, the query runs in 4.5 minutes, which is reasonable. I had planned to try Manfred's index correlation patch to see if it would give better estimates for an index scan. The index scan method took maybe 6.5x as long, but the estimate was that it would take 1400x as long. I think instead of trying out his patch I might actually work on my application! Ken ---(end of broadcast)--- TIP 7: don't forget to increase your free space map settings
Re: [PERFORM] bad estimates
Ken Geis [EMAIL PROTECTED] writes: From what I've learned, we want to convince the optimizer to use a table scan; that's a good thing. I want it to use hashed aggregates, but I can't convince it to (unless maybe I removed all of the statistics.) You probably just need to increase sort_mem. Multiple aggregates take more RAM to process in a hashtable style ... regards, tom lane ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
Re: [PERFORM] bad estimates / non-scanning aggregates
On Thu, Aug 28, 2003 at 17:10:31 -0700, Ken Geis [EMAIL PROTECTED] wrote: The query I want to run is select stock_id, min(price_date) from day_ends group by stock_id; The fast way to do this is: select distinct on (stock_id) stock_id, price_date order by stock_id, price_date; Also, to get the MIN for a given group, not all values of the index need to be seen. Must pgsql do a full scan because it treats all aggregates in the same way? Are MIN and MAX used often enough to justify special treatment, and could that be cleanly implemented? Perhaps the aggregate function can request the data in a certain order, be told that it is being passed data in a certain order, and return before seeing the entire set of data. Yes, max and min are not treated special so they don't benefit from indexes. This has been discussed repeatedly in the archives. ---(end of broadcast)--- TIP 4: Don't 'kill -9' the postmaster
Re: [PERFORM] bad estimates / non-scanning aggregates
Bruno Wolff III wrote: On Thu, Aug 28, 2003 at 17:10:31 -0700, Ken Geis [EMAIL PROTECTED] wrote: The query I want to run is select stock_id, min(price_date) from day_ends group by stock_id; The fast way to do this is: select distinct on (stock_id) stock_id, price_date order by stock_id, price_date; Not according to the optimizer! Plus, this is not guaranteed to return the correct results. Unique (cost=3711244.30..3795942.42 rows=6366 width=8) - Sort (cost=3711244.30..3753593.36 rows=16939624 width=8) Sort Key: stock_id, price_date - Seq Scan on day_ends (cost=0.00..361892.24 rows=16939624 width=8) ---(end of broadcast)--- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faqs/FAQ.html
Re: [PERFORM] bad estimates / non-scanning aggregates
Bruno Wolff III wrote: Not according to the optimizer! Plus, this is not guaranteed to return the correct results. For it to be fast you need an index on (stock_id, price_date) so that you can use an index scan. I already said that such an index existed. In fact, it is the primary key of the table. And yes, I *am* analyzed! The answers are guarenteed to be correct. See: http://developer.postgresql.org/docs/postgres/sql-select.html#SQL-DISTINCT That's good to know. Thanks! Ken ---(end of broadcast)--- TIP 2: you can get off all lists at once with the unregister command (send unregister YourEmailAddressHere to [EMAIL PROTECTED])
Re: [PERFORM] bad estimates
Bruno Wolff III wrote: On Thu, Aug 28, 2003 at 20:00:32 -0700, Ken Geis [EMAIL PROTECTED] wrote: Bruno Wolff III wrote: Not according to the optimizer! Plus, this is not guaranteed to return the correct results. For it to be fast you need an index on (stock_id, price_date) so that you can use an index scan. I already said that such an index existed. In fact, it is the primary key of the table. And yes, I *am* analyzed! Your original example didn't actually match that of the table you are showing examples from. In that example the second half of the primary key was the date not the end of the day price. If this is the case for the real table, then that is the reason the distinct on doesn't help. I had obfuscated the table in the example and forgot to do the same with the query. Serves me right for thinking I care about that. A big problem is that the values I am working with are *only* the primary key and the optimizer is choosing a table scan over an index scan. That is why I titled the email bad estimates. The table has (stock_id, price_date) as the primary key, and a bunch of other columns. What I *really* want to do efficiently is select stock_id, min(price_date), max(price_date) from day_ends group by stock_id; It is not the table or the query that is wrong. It is either the db parameters or the optimizer itself. Ken ---(end of broadcast)--- TIP 8: explain analyze is your friend
Re: [PERFORM] bad estimates
On Thu, Aug 28, 2003 at 20:46:00 -0700, Ken Geis [EMAIL PROTECTED] wrote: A big problem is that the values I am working with are *only* the primary key and the optimizer is choosing a table scan over an index scan. That is why I titled the email bad estimates. The table has (stock_id, price_date) as the primary key, and a bunch of other columns. What I *really* want to do efficiently is select stock_id, min(price_date), max(price_date) from day_ends group by stock_id; It is not the table or the query that is wrong. It is either the db parameters or the optimizer itself. If you want both the max and the min, then things are going to be a bit more work. You are either going to want to do two separate selects or join two selects or use subselects. If there aren't enough prices per stock, the sequential scan might be fastest since you only need to go through the table once and don't have to hit the index blocks. It is still odd that you didn't get a big speed up for just the min though. You example did have the stock id and the date as the primary key which would make sense since the stock id and stock price on a day wouldn't be guarenteed to be unique. Are you absolutely sure you have a combined key on the stock id and the stock price? ---(end of broadcast)--- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faqs/FAQ.html
Re: [PERFORM] bad estimates
Bruno Wolff III wrote: Can you do a \d on the real table or is that too sensitive? It was silly of me to think of this as particularly sensitive. stocks= \d day_ends Table public.day_ends Column | Type | Modifiers +--+--- stock_id | integer | not null price_date | date | not null open | numeric(9,4) | high | numeric(9,4) | low| numeric(9,4) | close | numeric(9,4) | volume | integer | Indexes: day_ends_pkey primary key btree (stock_id, price_date) Triggers: RI_ConstraintTrigger_16558399 It still doesn't make sense that you have a primary key that is a stock and its price. What happens when the stock has the same price on two different dates? And I doubt that you are looking for the minimum and maximum dates for which you have price data. So it is hard to believe that the index for your primary key is the one you need for your query. I can see the naming being confusing. I used price_date because, of course, date is not a legal name. day_ends is a horrible name for the table; daily_bars would probably be better. I *am* looking for the mininum and maximum dates for which I have price data. I'm running this query to build a chart so I can see visually where the majority of my data begins to use as the start of a window for analysis. When run on 7.3.3, forcing an index scan by setting enable_seqscan=false, the query took 55 minutes to run. The index is about 660M in size, and the table is 1G. As I mentioned before, with table scans enabled, it bombs, running out of temporary space. Hey Bruno, thanks for your attention here. I'm not a newbie, but I've never really had performance issues with pgsql before. And I've been running this database for a couple of years now, but I haven't run these queries against it. Ken ---(end of broadcast)--- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PERFORM] bad estimates
Ken Geis wrote: I went through the code (7.4 beta2) that estimates the cost of an index scan path. What I need to be sure of is that when running a query in pgsql that uses only the columns that are in an index, the underlying table need not be accessed. I know that Oracle does this. Thinking about it some more, it's obvious to me that a pgsql index scan must be accessing the underlying table even though all of the information needed is in the index itself. A linear scan of a 660M file should not take 55 minutes. I could confirm this with stats, but someone out there probably already knows the answer here. Ken ---(end of broadcast)--- TIP 6: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] bad estimates
I went through the code (7.4 beta2) that estimates the cost of an index scan path. What I need to be sure of is that when running a query in pgsql that uses only the columns that are in an index, the underlying table need not be accessed. I know that Oracle does this. PostgreSQL absolutely does not do this. It is also not possible to do this due to MVCC. Chris ---(end of broadcast)--- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PERFORM] bad estimates
Ken Geis wrote: When run on 7.3.3, forcing an index scan by setting enable_seqscan=false, the query took 55 minutes to run. The index is about 660M in size, and the table is 1G. As I mentioned before, with table scans enabled, it bombs, running out of temporary space. Man, I should wait a while before I send mails, because I keep having more to say! Some good news here. Doing the same as above on 7.4beta2 took 29 minutes. Now, the 7.3.3 was on reiser and 7.4 on ext2, so take that as you will. 7.4's index selectivity estimate seems much better; 7.3.3's anticipated rows was ten times the actual; 7.4's is one half of the actual. Ken ---(end of broadcast)--- TIP 8: explain analyze is your friend
Re: [PERFORM] bad estimates
On Fri, 29 Aug 2003, Ken Geis wrote: Some good news here. Doing the same as above on 7.4beta2 took 29 minutes. Now, the 7.3.3 was on reiser and 7.4 on ext2, so take that as you will. 7.4's index selectivity estimate seems much better; 7.3.3's anticipated rows was ten times the actual; 7.4's is one half of the actual. Min() Max() unfortunatly suck on PG. It will be that way for a while perhaps at some point someone will make a special case and convince -HACKERS it is a Good Thing(tm) (Like select count(*) from table being 'cached' - a lot of people probably get bad first impressions because of that) Would it be possible ot rewrite your queries replacing min/max with a select stock_id from bigtable where blah = blorch order by stock_id (desc|asc) limit 1? because that would enable PG to use an index and magically go fast. You may need a subselect.. -- Jeff Trout [EMAIL PROTECTED] http://www.jefftrout.com/ http://www.stuarthamm.net/ ---(end of broadcast)--- TIP 4: Don't 'kill -9' the postmaster
Re: [PERFORM] bad estimates
On Fri, 29 Aug 2003, Ken Geis wrote: Ken Geis wrote: I went through the code (7.4 beta2) that estimates the cost of an index scan path. What I need to be sure of is that when running a query in pgsql that uses only the columns that are in an index, the underlying table need not be accessed. I know that Oracle does this. Thinking about it some more, it's obvious to me that a pgsql index scan must be accessing the underlying table even though all of the information needed is in the index itself. A linear scan of a 660M file should not take 55 minutes. I could confirm this with stats, but someone out there probably already knows the answer here. Unfortunately not all the information needed is in the index. You can't tell from the index alone currently whether or not the row is visible to you. Adding said information would be possible but there are downsides to that as well (there are some past discussions on the topic, but I'm too lazy to look them up to give a link, check the archives ;) ). ---(end of broadcast)--- TIP 2: you can get off all lists at once with the unregister command (send unregister YourEmailAddressHere to [EMAIL PROTECTED])
Re: [PERFORM] bad estimates
Bruno Wolff III wrote: If you want both the max and the min, then things are going to be a bit more work. You are either going to want to do two separate selects or join two selects or use subselects. If there aren't enough prices per stock, the sequential scan might be fastest since you only need to go through the table once and don't have to hit the index blocks. It is still odd that you didn't get a big speed up for just the min though. I found I'm suffering from an effect detailed in a previous thread titled Does correlation mislead the optimizer on large tables? Ken ---(end of broadcast)--- TIP 8: explain analyze is your friend
Re: [PERFORM] bad estimates
If you want both the max and the min, then things are going to be a bit more work. You are either going to want to do two separate selects or join two selects or use subselects. If there aren't enough prices per stock, the sequential scan might be fastest since you only need to go through the table once and don't have to hit the index blocks. It is still odd that you didn't get a big speed up for just the min though. I found I'm suffering from an effect detailed in a previous thread titled Does correlation mislead the optimizer on large tables? I don't know about large tables, but this is a big problem and something I'm going to spend some time validating later today. I think Manfred's patch is pretty good and certainly better than where we are but I haven't used it yet to see if it's the magic ticket for many of these index problems. -sc -- Sean Chittenden ---(end of broadcast)--- TIP 7: don't forget to increase your free space map settings
