Re: Additional improvements to extended statistics

On Sun, Mar 15, 2020 at 1:08 PM Tomas Vondra
<tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> On Sat, Mar 14, 2020 at 05:56:10PM +0100, Tomas Vondra wrote:
> >Attached is a patch series rebased on top of the current master, after
> >committing the ScalarArrayOpExpr enhancements. I've updated the OR patch
> >to get rid of the code duplication, and barring objections I'll get it
> >committed shortly together with the two parts improving test coverage.
> >
>
> I've pushed the two patches improving test coverage for functional
> dependencies and MCV lists, which seems mostly non-controversial. I'll
> wait a bit more with the two patches actually changing behavior (rebased
> version attached, to keep cputube happy).

Some comment fixes:

- /* Check if the expression the right shape (one Var,
one Const) */
- if (!examine_clause_args(expr->args, &var, NULL, NULL))
+ /*
+ * Check if the expression the right shape (one Var
and one Const,
+ * or two Vars).
+ */

Check if the expression "has" or "is of" the right shape.

- * Attempts to match the arguments to either (Var op Const) or (Const op Var),
- * possibly with a RelabelType on top. When the expression matches this form,
- * returns true, otherwise returns false.
+ * Attempts to match the arguments to either (Var op Const) or (Const op Var)
+ * or (Var op Var), possibly with a RelabelType on top. When the expression
+ * matches this form, returns true, otherwise returns false.

... match the arguments to (Var op Const), (Const op Var) or (Var op Var), ...

+ /*
+ * Both variables have to be for the same relation
(otherwise it's
+ * a join clause, and we don't deal with those yet.
+ */

Missing close parenthesis.

Stimulated by some bad plans involving JSON, I found my way to your
WIP stats-on-expressions patch in this thread. Do I understand
correctly that it will eventually also support single expressions,
like CREATE STATISTICS t_distinct_abc (ndistinct) ON
(my_jsonb_column->>'abc') FROM t? It looks like that would solve
problems that otherwise require a generated column or an expression
index just to get ndistinct.