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Restore preprocess_groupclause()

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0452b461bc made optimizer explore alternative orderings of group-by pathkeys.
It eliminated preprocess_groupclause(), which was intended to match items
between GROUP BY and ORDER BY.  Instead, get_useful_group_keys_orderings()
function generates orderings of GROUP BY elements at the time of grouping
paths generation.  The get_useful_group_keys_orderings() function takes into
account 3 orderings of GROUP BY pathkeys and clauses: original order as written
in GROUP BY, matching ORDER BY clauses as much as possible, and matching the
input path as much as possible.  Given that even before 0452b461b,
preprocess_groupclause() could change the original order of GROUP BY clauses
we don't need to consider it apart from ordering matching ORDER BY clauses.

This commit restores preprocess_groupclause() to provide an ordering of
GROUP BY elements matching ORDER BY before generation of paths.  The new
version of preprocess_groupclause() takes into account an incremental sort.
The get_useful_group_keys_orderings() function now takes into 2 orderings of
GROUP BY elements: the order generated preprocess_groupclause() and the order
matching the input path as much as possible.

Discussion: https://postgr.es/m/CAPpHfdvyWLMGwvxaf%3D7KAp-z-4mxbSH8ti2f6mNOQv5metZFzg%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Andrei Lepikhov, Pavel Borisov
This commit is contained in:
Alexander Korotkov
2024-06-06 13:44:34 +03:00
parent 0c1af2c35c
commit 505c008ca3
4 changed files with 108 additions and 67 deletions
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6
src/include/nodes/pathnodes.h
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@@ -426,7 +426,11 @@ struct PlannerInfo
* items to be proven redundant, implying that there is only one group
* containing all the query's rows. Hence, if you want to check whether
* GROUP BY was specified, test for nonempty parse->groupClause, not for
* nonempty processed_groupClause.
* nonempty processed_groupClause. Optimizer chooses specific order of
* group-by clauses during the upper paths generation process, attempting
* to use different strategies to minimize number of sorts or engage
* incremental sort. See preprocess_groupclause() and
* get_useful_group_keys_orderings() for details.
*
* Currently, when grouping sets are specified we do not attempt to
* optimize the groupClause, so that processed_groupClause will be