Richard Huxton wrote:Can you post the output from your "explain analyse" calls too? The statistics aren't going to be the same on different machines.
Sure, here it is.
Thanks. (PS - remember to cc the list too).
EXPLAIN ANALYZE SELECT * FROM v;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Nested Loop (cost=0.00..263.12 rows=116 width=20) (actual time=5.171..109.910 rows=1020 loops=1)
Join Filter: (("inner"."version" = "outer"."version") OR ("inner".approved IS NOT NULL))
-> Seq Scan on users u (cost=0.00..1.01 rows=1 width=12) (actual time=0.005..0.009 rows=1 loops=1)
-> Index Scan using test_ on test t (cost=0.00..155.74 rows=7092 width=20) (actual time=0.012..64.873 rows=21000 loops=1)
Index Cond: (t.datum <= "outer".datum)
Total runtime: 111.879 ms
EXPLAIN ANALYZE SELECT t.* FROM test AS t INNER JOIN users AS u ON t.datum <= u.datum AND (t.version = '999' OR t.approved IS NOT NULL);
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Nested Loop (cost=0.00..7.78 rows=133 width=20) (actual time=0.035..7.733 rows=1020 loops=1)
-> Seq Scan on users u (cost=0.00..1.01 rows=1 width=8) (actual time=0.006..0.010 rows=1 loops=1)
-> Index Scan using test_999 on test t (cost=0.00..5.11 rows=132 width=20) (actual time=0.017..3.358 rows=1020 loops=1)
Index Cond: (t.datum <= "outer".datum)
Filter: (("version" = 999) OR (approved IS NOT NULL))
Total runtime: 9.528 ms
OK - so what you want to know is why index "test_999" is used in the second but not the first, even though both return the same rows.
The fact is that the conditional index:
CREATE INDEX test_999 ON test (datum)
WHERE version = '999' OR approved IS NOT NULL;
AFAIK looks at the WHERE clause of your query to determine where it can run. Don't forget that the planner needs to pick which index is best *before* it starts fetching data.
So - in the first example there might be rows where e.g. t.version=998 which means test_999 would be a poor choice of index.
-- Richard Huxton Archonet Ltd
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