Am Freitag, 28. Dezember 2007 schrieb Tom Lane: > Peter Eisentraut <[EMAIL PROTECTED]> writes: > > I have been observing a case where the row count estimation for LIKE > > 'foo' is (much) higher than for LIKE 'foo%', the rest of the query being > > the same. This is a special case of the estimation for equality being > > higher than for a range query that includes the value used in the > > equality. > > Not really --- LIKE estimation is only weakly related to range > estimation.
Here is a narrowed down example. * Pattern search EXPLAIN ANALYZE SELECT id FROM person WHERE lower(person.name) LIKE 'foo%' AND person.follow_nr=0 AND person.person_type='P' AND person.batch_nr=0; Index Scan using person_idx_3 on person (cost=0.01..6.03 rows=1 width=8) (actual time=0.276..4.917 rows=188 loops=1) Index Cond: ((lower((name)::text) ~>=~ 'foo'::text) AND (lower((name)::text) ~<~ 'fop'::text) AND (person_type = 'P'::bpchar) AND (batch_nr = 0) AND (follow_nr = 0)) Filter: (lower((name)::text) ~~ 'foo%'::text) * Equality search EXPLAIN ANALYZE SELECT id FROM person WHERE lower(person.name) LIKE 'foo' AND person.follow_nr=0 AND person.person_type='P' AND person.batch_nr=0; Index Scan using person_idx_3 on person (cost=0.00..2527.84 rows=627 width=8) (actual time=0.043..0.072 rows=7 loops=1) Index Cond: ((lower((name)::text) ~=~ 'foo'::text) AND (person_type = 'P'::bpchar) AND (batch_nr = 0) AND (follow_nr = 0)) Filter: (lower((name)::text) ~~ 'foo'::text) So it expects 1 row for the pattern search and 627 rows for the equality search, which doesn't make mathematical sense. What I had meant earlier with range and equality estimation is that this is (presumably) about the same as guessing 1 row for (x >= 5 AND x < 6), but 627 rows for (x = 5). Somehow, these two estimation methods should be "talking" to each other. PostgreSQL is version 8.1.9. -- Peter Eisentraut http://developer.postgresql.org/~petere/ ---------------------------(end of broadcast)--------------------------- TIP 4: Have you searched our list archives? http://archives.postgresql.org
