Re: Fix to not check included columns in ANALYZE on indexes

On Sat, 30 Jun 2018 14:13:49 -0400
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Peter Geoghegan <pg(at)bowt(dot)ie> writes:
> > I think that the argument Tom is making is that it might be useful to
> > have statistics on the expression regardless of this -- the expression
> > may be interesting in some general sense. For example, one can imagine
> > the planner creating a plan with a hash aggregate rather than a group
> > aggregate, but only when statistics on an expression are available,
> > somehow.
>
> Right. For instance, "select sum(x) from ... group by y+z" is only
> suitable for hash aggregation if we can predict that there's a fairly
> small number of distinct values of y+z. This makes it useful to have
> stats on the expression y+z, independently of whether any related index
> actually gets used in the plan.

Thank you for your explanation. I understand the usefulness of the statistics
on non-key expression attributions and that "CREATE INDEX ... INCLUDE" migth be
a means to collect the statistics on "non-key" expressions in future.

>
> regards, tom lane
>

--
Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>