Re: Removing useless DISTINCT clauses

On 8 April 2018 at 08:55, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> regression=# create table t1 (a int,b int, c int, d int, primary key(a,b));
> CREATE TABLE
> regression=# explain verbose select distinct * from t1 order by a,c,b;
> server closed the connection unexpectedly
> This probably means the server terminated abnormally
> before or while processing the request.

Ouch!

> The reason is we're hitting this assert, a bit further down in
> create_distinct_paths:
>
> /* Assert checks that parser didn't mess up... */
> Assert(pathkeys_contained_in(root->distinct_pathkeys,
> needed_pathkeys));
>
>
> I don't think that that makes this patch unsalvageable. The way forward
> probably involves remembering that we removed some distinctClause items
> (so that we know the correspondence to the sortClause no longer holds)
> and then working harder in create_distinct_paths when that's the case.
>
> However, that's not something that's going to happen on the last day
> of the commitfest. So I'm going to mark this Returned With Feedback
> and encourage you to return to the matter in v12.
>
> In the meantime, attached is the version of the patch that I was about to
> commit before getting cold feet. It has some cosmetic improvements
> over yours, notably comment additions in allpaths.c.

Thanks a lot for spending time on this.

I'll no doubt have some time over this coming winter to see if it can
be fixed and re-submitted in the PG12 cycle.

--
David Rowley http://www.2ndQuadrant.com/
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