Re: Expression index ignores column statistics target

Michael Fuhr <mike(at)fuhr(dot)org> writes:
> I've noticed that row count estimates for expression indexes appear
> to rely on default_statistics_target rather than on a column's
> actual statistics target. That is, if I use ALTER TABLE SET
> STATISTICS to increase a column's statistics target and then run
> ANALYZE, then estimates for non-expression-index queries improve
> as expected. However, queries that use an expression index remain
> accurate for only around the N most common values, where N is the
> default_statistics_target that was in effect when ANALYZE ran.

The code does in fact honor per-column statistics targets attached to
expression indexes, viz

alter table myfuncindex alter column pg_expression_1 set statistics 100;

This isn't documented, mainly because pg_dump doesn't promise to dump
such things, which it doesn't do because I didn't want to see the
"pg_expression_N" naming for expression index columns become graven on
stone tablets. I seem to recall bringing up the question of whether
we could find a less implementation-specific way of commanding this
behavior, but I can't find it in the archives right now.

regards, tom lane