Marcio Ribeiro <mribeiro@xxxxxxxxx> writes: > Short question: > Why would pg optimizer choose a worst (slower) query plan for a > query with 'LIMIT 1' instead of, say, 'LIMIT 3'? > Complete scenario: > Query: 'SELECT * FROM a WHERE a.b_id = 42 ORDER BY created LIMIT 1' > - b_id is a FK to b; > - created is a datetime with the time of the creation of the row; > - both 'b' and 'created' are indexed separately > This query, with the LIMIT 1, uses the index on created, which is much > slower (10x) than if it used the index on b_id It's trying to avoid a sort; or to be less anthropomorphic, the estimated cost of scanning the "created" index until it hits the first row with b_id=42 is less than the estimated cost of collecting all the rows with b_id=42 and then sorting them by "created". The estimates unfortunately are kind of shaky because it's hard to predict how many rows will get skipped before finding one with b_id=42. If you do this type of query often enough to care about its performance, you could consider creating a two-column index on (b_id, created) (in that order). regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
