On Sat, 30 Jun 2018 14:13:49 -0400
Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Peter Geoghegan <pg@bowt.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@sraoss.co.jp>
