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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 aa97890b6e
commit 336879f555
2 changed files with 8 additions and 3 deletions
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8
src/backend/optimizer/path/equivclass.c
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@ -795,9 +795,11 @@ find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel)
} }
/* /*
* Find an equivalence class member expression that can be safely used by a * Find an equivalence class member expression that can be safely used to build
* sort node on top of the provided relation. The rules here must match those * a sort node using the provided relation. The rules are a subset of those
* applied in prepare_sort_from_pathkeys. * 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 * Expr *
find_em_expr_usable_for_sorting_rel(PlannerInfo *root, EquivalenceClass *ec, find_em_expr_usable_for_sorting_rel(PlannerInfo *root, EquivalenceClass *ec,

3
src/backend/optimizer/plan/createplan.c
View File

@ -5844,6 +5844,9 @@ make_incrementalsort(Plan *lefttree, int numCols, int nPresortedCols,
* *
* Returns the node which is to be the input to the Sort (either lefttree, * Returns the node which is to be the input to the Sort (either lefttree,
* or a Result stacked atop 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 * static Plan *
prepare_sort_from_pathkeys(Plan *lefttree, List *pathkeys, prepare_sort_from_pathkeys(Plan *lefttree, List *pathkeys,