Hello, A table has two columns id and EffectiveId. First is primary key. EffectiveId is almost always equal to id (95%) unless records are merged. Many queries have id = EffectiveId condition. Both columns are very distinct and Pg reasonably decides that condition has very low selectivity and picks sequence scan. Simple perl script that demonstrates estimation error: https://gist.github.com/1356744 Estimation is ~200 times off (5 vs 950), for real situation it's very similar. Understandably difference depends on correlation coefficient. In application such wrong estimation result in seq scan of this table winning leading position in execution plans over other tables and index scans. What can I do to avoid this problem? Tested with PostgreSQL 9.0.3 on x86_64-apple-darwin10.6.0, compiled by GCC i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664), 64-bit -- Best regards, Ruslan. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
