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