Re: Expression index ignores column statistics target

Tom Lane wrote:
> 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.

Is this a TODO?

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