Abhijit Menon-Sen <ams@xxxxxxxx> writes: > The header_fields table contains 13.5M rows, of which only ~250K match > the where condition. I created an index like this: > create index hffpv on header_fields(field) > where field<=12 and (part!='' or value ilike '%,%') > Note the 2M estimated rowcount in the bitmap index scan on header_fields > vs. the actual number (264180). I think this is basically a lack-of-column-correlation-stats problem. The planner is estimating this on the basis of the overall selectivity of the "field<=12" condition, but it seems that "field<=12" is true for a much smaller fraction of the rows satisfying (part!='' or value ilike '%,%') than for the general population of rows in the header_fields table. There's been some speculation about obtaining stats on partial indexes as a substitute for solving the general problem of correlation stats, but I for one don't have a very clear understanding of how it'd work. regards, tom lane
