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Improve find_em_expr_usable_for_sorting_rel comment

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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
This commit is contained in:
Tomas Vondra
2020-12-22 02:00:39 +01:00
parent 9aff4dc01f
commit 1ca2eb1031
2 changed files with 8 additions and 3 deletions
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8
src/backend/optimizer/path/equivclass.c
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@@ -798,9 +798,11 @@ 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 rules are a subset of those
* applied in prepare_sort_from_pathkeys since that function deals with sorts
* that must be delayed until the last stages of query execution, while here
* we only care about proactive sorts.
*/
Expr *
find_em_expr_usable_for_sorting_rel(PlannerInfo *root, EquivalenceClass *ec,