Re: enable_incremental_sort changes query behavior

On Fri, Oct 02, 2020 at 09:19:44AM -0400, James Coleman wrote:
>
> ...
>
>I've been able to confirm that the problem goes away if we stop adding
>the gather merge paths in generate_useful_gather_paths().
>
>I'm not sure yet what conclusion that leads us to. It seems to be that
>the biggest clue remains that all of this works correctly unless one
>of the selected columns (which happens to be a pathkey at this point
>because it's a DISTINCT query) contains a volatile expression.
>

Yeah. It seems to me this is a bug in get_useful_pathkeys_for_relation,
which is calling find_em_expr_for_rel and is happy with anything it
returns. But this was copied from postgres_fdw, which however does a bit
more here:

if (pathkey_ec->ec_has_volatile ||
!(em_expr = find_em_expr_for_rel(pathkey_ec, rel)) ||
!is_foreign_expr(root, rel, em_expr))

So not only does it explicitly check volatility of the pathkey, it also
calls is_foreign_expr which checks the expression for mutable functions.

The attached patch seems to fix this, but it only adds the check for
mutable functions. Maybe it should check ec_has_volatile too ...

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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