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Properly prepare varinfos in estimate_multivariate_bucketsize()

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To estimate with extended statistics, we need to clear the varnullingrels
field in the expression, and duplicates are not allowed in the GroupVarInfo
list.  We might re-use add_unique_group_var(), but we don't do so for two
reasons.

  1) We must keep the origin_rinfos list ordered exactly the same way as
     varinfos.
  2) add_unique_group_var() is designed for estimate_num_groups(), where a
     larger number of groups is worse.   While estimating the number of hash
     buckets, we have the opposite: a lesser number of groups is worse.
     Therefore, we don't have to remove "known equal" vars: the removed var
     may valuably contribute to the multivariate statistics to grow the number
     of groups.

This commit adds custom code to estimate_multivariate_bucketsize() to
initialize varinfos properly.

Reported-by: Robins Tharakan <tharakan@gmail.com>
Discussion: https://postgr.es/m/18885-da51324078588253%40postgresql.org
Author: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
This commit is contained in:
Alexander Korotkov
2025-04-23 20:13:51 +03:00
parent 3db61db48e
commit 9f404d7922
3 changed files with 96 additions and 3 deletions
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60
src/backend/utils/adt/selfuncs.c
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@@ -3850,6 +3850,8 @@ estimate_multivariate_bucketsize(PlannerInfo *root, RelOptInfo *inner,
if (bms_get_singleton_member(relids, &relid) &&
root->simple_rel_array[relid]->statlist != NIL)
{
bool is_duplicate = false;
/*
* This inner-side expression references only one relation.
* Extended statistics on this clause can exist.
@@ -3880,11 +3882,61 @@ estimate_multivariate_bucketsize(PlannerInfo *root, RelOptInfo *inner,
*/
continue;
varinfo = (GroupVarInfo *) palloc(sizeof(GroupVarInfo));
/*
* We're going to add the new clause to the varinfos list. We
* might re-use add_unique_group_var(), but we don't do so for
* two reasons.
*
* 1) We must keep the origin_rinfos list ordered exactly the
* same way as varinfos.
*
* 2) add_unique_group_var() is designed for
* estimate_num_groups(), where a larger number of groups is
* worse. While estimating the number of hash buckets, we
* have the opposite: a lesser number of groups is worse.
* Therefore, we don't have to remove "known equal" vars: the
* removed var may valuably contribute to the multivariate
* statistics to grow the number of groups.
*/
/*
* Clear nullingrels to correctly match hash keys. See
* add_unique_group_var()'s comment for details.
*/
expr = remove_nulling_relids(expr, root->outer_join_rels, NULL);
/*
* Detect and exclude exact duplicates from the list of hash
* keys (like add_unique_group_var does).
*/
foreach(lc1, varinfos)
{
varinfo = (GroupVarInfo *) lfirst(lc1);
if (!equal(expr, varinfo->var))
continue;
is_duplicate = true;
break;
}
if (is_duplicate)
{
/*
* Skip exact duplicates. Adding them to the otherclauses
* list also doesn't make sense.
*/
continue;
}
/*
* Initialize GroupVarInfo. We only use it to call
* estimate_multivariate_ndistinct(), which doesn't care about
* ndistinct and isdefault fields. Thus, skip these fields.
*/
varinfo = (GroupVarInfo *) palloc0(sizeof(GroupVarInfo));
varinfo->var = expr;
varinfo->rel = root->simple_rel_array[relid];
varinfo->ndistinct = 0.0;
varinfo->isdefault = false;
varinfos = lappend(varinfos, varinfo);
/*
@@ -3894,8 +3946,10 @@ estimate_multivariate_bucketsize(PlannerInfo *root, RelOptInfo *inner,
origin_rinfos = lappend(origin_rinfos, rinfo);
}
else
{
/* This clause can't be estimated with extended statistics */
otherclauses = lappend(otherclauses, rinfo);
}
clauses = foreach_delete_current(clauses, lc);
}