Re: PoC/WIP: Extended statistics on expressions

On Mon, Aug 16, 2021 at 05:41:57PM +0200, Tomas Vondra wrote:
> > This still seems odd:
> >
> > postgres=# CREATE STATISTICS asf ON i FROM t;
> > ERROR: extended statistics require at least 2 columns
> > postgres=# CREATE STATISTICS asf ON (i) FROM t;
> > CREATE STATISTICS
> >
> > It seems wrong that the command works with added parens, but builds expression
> > stats on a simple column (which is redundant with what analyze does without
> > extended stats).
>
> Well, yeah. But I think this is a behavior that was discussed somewhere in
> this thread, and the agreement was that it's not worth the complexity, as
> this comment explains
>
> * XXX We do only the bare minimum to separate simple attribute and
> * complex expressions - for example "(a)" will be treated as a complex
> * expression. No matter how elaborate the check is, there'll always be
> * a way around it, if the user is determined (consider e.g. "(a+0)"),
> * so it's not worth protecting against it.
>
> Patch 0001 fixes the "double parens" issue discussed elsewhere in this
> thread, and patch 0002 tweaks CREATE STATISTICS to treat "(a)" as a simple
> column reference.

0002 refuses to create expressional stats on a simple column reference like
(a), which I think is helps to avoid a user accidentally creating useless ext
stats objects (which are redundant with the table's column stats).

0002 does not attempt to refuse cases like (a+0), which I think is fine:
we don't try to reject useless cases if someone insists on it.
See 240971675, 701fd0bbc.

So I am +1 to apply both patches.

I added this as an Opened Item for increased visibility.

--
Justin