From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: James Coleman <jtc331@gmail.com>
Cc: Jaime Casanova <jaime.casanova@2ndquadrant.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: enable_incremental_sort changes query behavior
Date: Fri, 2 Oct 2020 16:32:07 +0200
Message-ID: <20201002143207.qt7re34mhsfmfi6f@development> (raw)
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<CAAaqYe_Ru5h2NxhGtERfj1RGskyjAg8PfG1XYTLRYiUmqNGEXw@mail.gmail.com>
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<CAAaqYe9QBf_UWT+-_SrMs4iHKbUC_WkaHvJjthJuqrmvW2UdZw@mail.gmail.com>
<20201001220822.rwwxajwzobljgqwz@development>
<CAAaqYe-wssm0qqc0sDm7+5mNTAUYKfXx9pnGcD7vCS3g49Gmmw@mail.gmail.com>
<CAAaqYe83Ec57fqJiA0fkEGQ-PUk_VwiR2nHbg=LHPwiG5jtXJQ@mail.gmail.com>
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
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.cindex b399592ff8..33df9b0783 100644--- a/src/backend/optimizer/path/allpaths.c+++ b/src/backend/optimizer/path/allpaths.c@@ -2771,6 +2771,7 @@ get_useful_pathkeys_for_relation(PlannerInfo *root, RelOptInfo *rel)
{
PathKey *pathkey = (PathKey *) lfirst(lc);
EquivalenceClass *pathkey_ec = pathkey->pk_eclass;
+ Expr *em_expr;
/*
* We can only build an Incremental Sort for pathkeys which
@@ -2783,7 +2784,8 @@ get_useful_pathkeys_for_relation(PlannerInfo *root, RelOptInfo *rel)
* enable not just an incremental sort on the entirety of
* query_pathkeys but also incremental sort below a JOIN.
*/
- if (!find_em_expr_for_rel(pathkey_ec, rel))+ if (!(em_expr = find_em_expr_for_rel(pathkey_ec, rel)) ||+ contain_mutable_functions((Node *) em_expr))
break;
npathkeys++;
Attachments:
[text/plain] incremental-sort-fix.patch (924B, ../20201002143207.qt7re34mhsfmfi6f@development/2-incremental-sort-fix.patch)
download | inline diff:diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.cindex b399592ff8..33df9b0783 100644--- a/src/backend/optimizer/path/allpaths.c+++ b/src/backend/optimizer/path/allpaths.c@@ -2771,6 +2771,7 @@ get_useful_pathkeys_for_relation(PlannerInfo *root, RelOptInfo *rel)
{
PathKey *pathkey = (PathKey *) lfirst(lc);
EquivalenceClass *pathkey_ec = pathkey->pk_eclass;
+ Expr *em_expr;
/*
* We can only build an Incremental Sort for pathkeys which
@@ -2783,7 +2784,8 @@ get_useful_pathkeys_for_relation(PlannerInfo *root, RelOptInfo *rel)
* enable not just an incremental sort on the entirety of
* query_pathkeys but also incremental sort below a JOIN.
*/
- if (!find_em_expr_for_rel(pathkey_ec, rel))+ if (!(em_expr = find_em_expr_for_rel(pathkey_ec, rel)) ||+ contain_mutable_functions((Node *) em_expr))
break;
npathkeys++;
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To: pgsql-hackers@postgresql.org
Cc: tomas.vondra@2ndquadrant.com, jtc331@gmail.com, jaime.casanova@2ndquadrant.com
Subject: Re: enable_incremental_sort changes query behavior
In-Reply-To: <20201002143207.qt7re34mhsfmfi6f@development>
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