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.
regards, tom lane