Re: enable_incremental_sort changes query behavior
От | Tomas Vondra |
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Тема | Re: enable_incremental_sort changes query behavior |
Дата | |
Msg-id | 20201002143207.qt7re34mhsfmfi6f@development обсуждение исходный текст |
Ответ на | Re: enable_incremental_sort changes query behavior (James Coleman <jtc331@gmail.com>) |
Ответы |
Re: enable_incremental_sort changes query behavior
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Список | pgsql-hackers |
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 PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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