Ioana Danes wrote:
I have a problem with a query that in postgres 7.4 and 8.12 has an
acceptable response time but in postgres 8.14 is very slow.
This is the table I use:
*
create* *table* TEST (
TESTID INT8 *not* *null*,
TESTTYPE INT4 *null*,
*constraint* PK_TESTID *primary* *key* (TESTID));
*create* *index* IX_TEST_TESTTYPE *on* TEST (TESTTYPE);
And this is the query with the problem:
*explain select* *max*(TESTID) *from* TEST *where* TESTTYPE = 1577;
The query plan in postgres 7.4 and 8.12 is using the index by TESTTYPE
field, which is what I want in this case.
QUERY PLAN
Aggregate (cost=25.97..25.97 rows=1 width=8)
-> Index Scan using ix_test_testtype on test (cost=0.00..25.95
rows=9 width=8)
Index Cond: (testtype = 1577)
With postgres 8.14 the query plan uses the primary key PK_TESTID with
filter by TESTTYPE, which it takes almost 10 minutes to execute:
QUERY PLAN
Limit (cost=0.00..41.46 rows=1 width=8)
-> Index Scan Backward using pk_testid on test (cost=…)
Filter: ((testid IS NOT NULL) and (testtype = 1577))
When replacing the index
*create* *index* IX_TEST_TESTTYPE *on* TEST (TESTTYPE);
with
*create* *index* IX_TEST_TESTTYPE *on* TEST (TESTTYPE, TESTID);
the query plan uses this index and the execution of this select is
extremely fast.
From what I can see, the query plan for 8.14 is using a index scan by
the field used with max() function with a filter by the field in where
condition.
Should not the query plan use an index scan by the field in where
condition (which in my case is a small range) and come up with the max
value in that range?
Is this a bug, am I missing a configuration step or this is how it is
supposed to work?
You've left out the best details. Post an 'explain analyze' from both
versions, and don't cut anything out :)
I'm guessing postgres is seeing an index on the table is faster because
it doesn't think you have many rows in the table. How many are there,
and have you done an analyze of the table after loading the data in?
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