Nis Jorgensen skrev:
> Paul Rauch wrote:
>
>> Is there a way to reduce execution time by counting objects?
>>
>> it's even splitted in two queries now, with only 2474487 entries ;)
>>
>> in django I use "model.objects.count()"
>>
>> SELECT COUNT(*) FROM "rainbowtables_hashes":34.552
>>
>> SELECT COUNT(*) FROM "rainbowtables_hashes":31.024
>>
>>
>
> I guess you are using postgresql. This is a known "issue" with that
> database - count(*) on an entire table (or large subsets) is slow (and
> takes time proportional to the rows found).
>
>
[snip]
> Hmm, i just realized a generic way of making this work in postgresql (if
> you are happy to do your own ddl). Basically use an updatable view,
> which maintains the additional table behind the scenes. Will write that
> up in more detail later.
>
If anyone is interested,. I did a little testing on this. It does seem
to work in some cases, but not using the query plan I thought it would.
My setup:
CREATE TABLE entity_base (id SERIAL PRIMARY KEY, f1 varchar(255), f2
varchar(255), f3 varchar(255), f4 varchar(255), f5 varchar(255), f6
varchar(255), f7 varchar(255));
CREATE TABLE entity_idx (id integer PRIMARY KEY REFERENCES entity_base
(id) ON DELETE CASCADE);
CREATE VIEW entity AS (SELECT * FROM entity_base NATURAL JOIN entity_idx );
-- Create one row of data
INSERT INTO entity_base (f1,f2,f3,f4,f5,f6,f7) VALUES ('Long random
string','must be random to avoid compression', ...);
-- Each line below doubles the amount of rows
INSERT INTO entity_base (f1,f2,f3,f4,f5,f6,f7) SELECT
f1,f2,f3,f4,f5,f6,f7 FROM entity_base;
INSERT INTO entity_base (f1,f2,f3,f4,f5,f6,f7) SELECT
f1,f2,f3,f4,f5,f6,f7 FROM entity_base;
-- ...
-- 21 times
-- ...
INSERT INTO entity_base (f1,f2,f3,f4,f5,f6,f7) SELECT
f1,f2,f3,f4,f5,f6,f7 FROM entity_base;
-- populate the "index table"
INSERT INTO entity_idx SELECT id FROM entity_base;
> EXPLAIN ANALYZE SELECT count(*) FROM entity_base;
QUERY
PLAN
-------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=183116.80..183116.81 rows=1 width=0) (actual
time=16005.803..16005.803 rows=1 loops=1)
-> Seq Scan on entity_base (cost=0.00..182461.44 rows=262144
width=0) (actual time=58.861..15893.200 rows=262144 loops=1)
Total runtime: 16005.852 ms
>EXPLAIN ANALYZE SELECT count(*) FROM entity;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=193688.18..193688.19 rows=1 width=0) (actual
time=2289.985..2289.986 rows=1 loops=1)
-> Merge Join (cost=0.00..193032.82 rows=262144 width=0) (actual
time=0.148..2178.405 rows=262144 loops=1)
Merge Cond: ("outer".id = "inner".id)
-> Index Scan using entity_base_pkey on entity_base
(cost=0.00..184324.58 rows=262144 width=4) (actual time=0.074..1253.894
rows=262144 loops=1)
-> Index Scan using entity_idx_pkey on entity_idx
(cost=0.00..4839.58 rows=262144 width=4) (actual time=0.067..291.596
rows=262144 loops=1)
Total runtime: 2290.056 ms
My hope was that the query could be satisfied by doing a seqscan on
entity_idx. This optimization is probably beyond the ability of
postgresql though. Apparently, however, the count can be satisfied
just by looking at the 2 indexes - which seems strange to me. I would
think at least one of the tables would need to be seqscanned, to ensure
that the rows have not been deleted.
What is missing above is creating INSERT/UPDATE/DELETE rules for the
"entity" view. I hacked together the ones below - they seem to work,
but there might be subtle points of how this works that I have not
understood.
CREATE RULE entity_delete as ON DELETE TO entity DO INSTEAD DELETE FROM
entity_base where id = old.id;
CREATE RULE entity_update as ON UPDATE TO entity DO INSTEAD UPDATE
entity_base SET f1=NEW.f1, f2=NEW.f2, f3=NEW.f3, f4=NEW.f4, f5=NEW.f5,
f6=NEW.f6, f7=NEW.f7 where id = NEW.id;
CREATE RULE entity_insert as ON INSERT TO entity DO INSTEAD (
INSERT INTO entity_base (f1,f2,f3,f4,f5,f6,f7) VALUES (NEW.f1,
NEW.f2, NEW.f3, NEW.f4, NEW.f5, NEW.f6, NEW.f7);
INSERT INTO entity_idx VALUES (currval('entity_base_id_seq'::regclass))
);
/Nis
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