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help / color / mirror / Atom feedFrom: Tomas Vondra <[email protected]>
Subject: [PATCH 5/5] Improve find_em_expr_usable_for_sorting_rel comment
Date: Wed, 16 Dec 2020 00:20:53 +0100
Clarify the relationship between find_em_expr_usable_for_sorting_rel and
prepare_sort_from_pathkeys, i.e. what restrictions need to be shared
between those two places.
Author: James Coleman
Reviewed-by: Tomas Vondra
Backpatch-through: 13
Discussion: https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs%3DhC0mSksZC%3DH5M8LSchj5e5OxpTAg%40mail.gmail.com
---
src/backend/optimizer/path/equivclass.c | 9 ++++++---
src/backend/optimizer/plan/createplan.c | 3 +++
2 files changed, 9 insertions(+), 3 deletions(-)
diff --git a/src/backend/optimizer/path/equivclass.c b/src/backend/optimizer/path/equivclass.c
index 70499af2cf..4466f880ee 100644
--- a/src/backend/optimizer/path/equivclass.c
+++ b/src/backend/optimizer/path/equivclass.c
@@ -798,9 +798,12 @@ find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel)
}
/*
- * Find an equivalence class member expression that can be safely used by a
- * sort node on top of the provided relation. The rules here must match those
- * applied in prepare_sort_from_pathkeys.
+ * Find an equivalence class member expression that can be safely used to build
+ * a sort node using the provided relation. The applied rules parallel those
+ * applied in prepare_sort_from_pathkeys but are a subset of that function since
+ * here we are concerned with proactive sorts while prepare_sort_from_pathkeys
+ * must deal with sorts that must be delayed until the last stages of query
+ * execution.
*/
Expr *
find_em_expr_usable_for_sorting_rel(PlannerInfo *root, EquivalenceClass *ec,
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 5ecf9f4065..f7a8dae3c6 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -5850,6 +5850,9 @@ make_incrementalsort(Plan *lefttree, int numCols, int nPresortedCols,
*
* Returns the node which is to be the input to the Sort (either lefttree,
* or a Result stacked atop lefttree).
+ *
+ * Note: Restrictions on what expressions are safely sortable may also need to
+ * be added to find_em_expr_usable_for_sorting_rel.
*/
static Plan *
prepare_sort_from_pathkeys(Plan *lefttree, List *pathkeys,
--
2.26.2
--------------DB02ACD71E58FCC25F7488B7--
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