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Reordering DISTINCT keys to match input path's pathkeys

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The ordering of DISTINCT items is semantically insignificant, so we
can reorder them as needed.  In fact, in the parser, we absorb the
sorting semantics of the sortClause as much as possible into the
distinctClause, ensuring that one clause is a prefix of the other.
This can help avoid a possible need to re-sort.

In this commit, we attempt to adjust the DISTINCT keys to match the
input path's pathkeys.  This can likewise help avoid re-sorting, or
allow us to use incremental-sort to save efforts.

For DISTINCT ON expressions, the parser already ensures that they
match the initial ORDER BY expressions.  When reordering the DISTINCT
keys, we must ensure that the resulting pathkey list matches the
initial distinctClause pathkeys.

This introduces a new GUC, enable_distinct_reordering, which allows
the optimization to be disabled if needed.

Author: Richard Guo
Reviewed-by: Andrei Lepikhov
Discussion: https://postgr.es/m/CAMbWs48dR26cCcX0f=8bja2JKQPcU64136kHk=xekHT9xschiQ@mail.gmail.com
This commit is contained in:
Richard Guo
2024-11-26 09:25:18 +09:00
parent 5b8728cd7f
commit a8ccf4e93a
11 changed files with 674 additions and 203 deletions
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38
src/backend/optimizer/path/pathkeys.c
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@@ -2207,6 +2207,41 @@ pathkeys_useful_for_grouping(PlannerInfo *root, List *pathkeys)
return n;
}
/*
* pathkeys_useful_for_distinct
* Count the number of pathkeys that are useful for DISTINCT or DISTINCT
* ON clause.
*
* DISTINCT keys could be reordered to benefit from the given pathkey list. As
* with pathkeys_useful_for_grouping, we return the number of leading keys in
* the list that are shared by the distinctClause pathkeys.
*/
static int
pathkeys_useful_for_distinct(PlannerInfo *root, List *pathkeys)
{
int n_common_pathkeys;
/*
* distinct_pathkeys may have become empty if all of the pathkeys were
* determined to be redundant. Return 0 in this case.
*/
if (root->distinct_pathkeys == NIL)
return 0;
/* walk the pathkeys and search for matching DISTINCT key */
n_common_pathkeys = 0;
foreach_node(PathKey, pathkey, pathkeys)
{
/* no matching DISTINCT key, we're done */
if (!list_member_ptr(root->distinct_pathkeys, pathkey))
break;
n_common_pathkeys++;
}
return n_common_pathkeys;
}
/*
* pathkeys_useful_for_setop
* Count the number of leading common pathkeys root's 'setop_pathkeys' in
@@ -2240,6 +2275,9 @@ truncate_useless_pathkeys(PlannerInfo *root,
if (nuseful2 > nuseful)
nuseful = nuseful2;
nuseful2 = pathkeys_useful_for_grouping(root, pathkeys);
if (nuseful2 > nuseful)
nuseful = nuseful2;
nuseful2 = pathkeys_useful_for_distinct(root, pathkeys);
if (nuseful2 > nuseful)
nuseful = nuseful2;
nuseful2 = pathkeys_useful_for_setop(root, pathkeys);