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Rename find_em_expr_usable_for_sorting_rel.

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I didn't particularly like this function name, as it fails to
express what's going on.  Also, returning the sort expression
alone isn't too helpful --- typically, a caller would also
need some other fields of the EquivalenceMember.  But the
sole caller really only needs a bool result, so let's make
it "bool relation_can_be_sorted_early()".

Discussion: https://postgr.es/m/91f3ec99-85a4-fa55-ea74-33f85a5c651f@swarm64.com
This commit is contained in:
Tom Lane
2021-04-20 11:37:36 -04:00
parent 3753982441
commit 7645376774
3 changed files with 28 additions and 27 deletions
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21
src/backend/optimizer/path/allpaths.c
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@@ -2697,20 +2697,19 @@ get_useful_pathkeys_for_relation(PlannerInfo *root, RelOptInfo *rel,
EquivalenceClass *pathkey_ec = pathkey->pk_eclass;
/*
* We can only build a sort for pathkeys which contain an EC
* member in the current relation's target, so ignore any suffix
* of the list as soon as we find a pathkey without an EC member
* in the relation.
* We can only build a sort for pathkeys that contain a
* safe-to-compute-early EC member computable from the current
* relation's reltarget, so ignore the remainder of the list as
* soon as we find a pathkey without such a member.
*
* By still returning the prefix of the pathkeys list that does
* meet criteria of EC membership in the current relation, we
* enable not just an incremental sort on the entirety of
* query_pathkeys but also incremental sort below a JOIN.
* It's still worthwhile to return any prefix of the pathkeys list
* that meets this requirement, as we may be able to do an
* incremental sort.
*
* If requested, ensure the expression is parallel safe too.
* If requested, ensure the sort expression is parallel-safe too.
*/
if (!find_em_expr_usable_for_sorting_rel(root, pathkey_ec, rel,
require_parallel_safe))
if (!relation_can_be_sorted_early(root, rel, pathkey_ec,
require_parallel_safe))
break;
npathkeys++;