Horton

Re: Wrong results with grouping sets

Richard Guo <guofenglinux@gmail.com>

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From: Richard Guo <guofenglinux@gmail.com>
To: PostgreSQL-development <pgsql-hackers@postgresql.org>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2023-10-07T10:29:41Z
Lists: pgsql-hackers

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  1. Remove the RTE_GROUP RTE if we drop the groupClause

  2. Mark expressions nullable by grouping sets

  3. Introduce an RTE for the grouping step

On Mon, Sep 25, 2023 at 3:11 PM Richard Guo <guofenglinux@gmail.com> wrote:

> I think the root cause is that when we generate distinct_pathkeys, we
> failed to realize that Var 'b' might be nullable by the grouping sets,
> so it's no longer always equal to Var 'a'.  It's not correct to deem
> that the PathKey for 'b' is redundant and thus remove it from the
> pathkeys list.
>
> We have the same issue when generating sort_pathkeys.  As a result, we
> may have the final output in the wrong order.  There were several
> reports about this issue before, such as [1][2].
>
> To fix this issue, I'm thinking that we mark the grouping expressions
> nullable by grouping sets with a dummy RTE for grouping sets, something
> like attached.
>

Hi Tom, I'm wondering if you've got a chance to look into this issue.
What do you think about the fix?

Thanks
Richard