From 96f990e23bae0559ede95e94340d2fc918dfde44 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 13 Jul 2011 20:23:09 -0400
Subject: [PATCH] Update some comments to clarify who does what in targetlist
creation.
No code changes; just avoid blaming query_planner for things it doesn't
really do.
---
src/backend/optimizer/plan/planner.c | 30 ++++++++++++++++------------
1 file changed, 17 insertions(+), 13 deletions(-)
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 98b673aca86..4ddb1211bb0 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -1248,17 +1248,17 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
need_sort_for_grouping = true;
/*
- * Always override query_planner's tlist, so that we don't
+ * Always override create_plan's tlist, so that we don't
* sort useless data from a "physical" tlist.
*/
need_tlist_eval = true;
}
/*
- * create_plan() returns a plan with just a "flat" tlist of
+ * create_plan returns a plan with just a "flat" tlist of
* required Vars. Usually we need to insert the sub_tlist as the
* tlist of the top plan node. However, we can skip that if we
- * determined that whatever query_planner chose to return will be
+ * determined that whatever create_plan chose to return will be
* good enough.
*/
if (need_tlist_eval)
@@ -1311,7 +1311,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
else
{
/*
- * Since we're using query_planner's tlist and not the one
+ * Since we're using create_plan's tlist and not the one
* make_subplanTargetList calculated, we have to refigure any
* grouping-column indexes make_subplanTargetList computed.
*/
@@ -2521,10 +2521,11 @@ choose_hashed_distinct(PlannerInfo *root,
* make_subplanTargetList
* Generate appropriate target list when grouping is required.
*
- * When grouping_planner inserts Aggregate, Group, or Result plan nodes
- * above the result of query_planner, we typically want to pass a different
- * target list to query_planner than the outer plan nodes should have.
- * This routine generates the correct target list for the subplan.
+ * When grouping_planner inserts grouping or aggregation plan nodes
+ * above the scan/join plan constructed by query_planner+create_plan,
+ * we typically want the scan/join plan to emit a different target list
+ * than the outer plan nodes should have. This routine generates the
+ * correct target list for the scan/join subplan.
*
* The initial target list passed from the parser already contains entries
* for all ORDER BY and GROUP BY expressions, but it will not have entries
@@ -2547,15 +2548,18 @@ choose_hashed_distinct(PlannerInfo *root,
* If we are grouping or aggregating, *and* there are no non-Var grouping
* expressions, then the returned tlist is effectively dummy; we do not
* need to force it to be evaluated, because all the Vars it contains
- * should be present in the output of query_planner anyway.
+ * should be present in the "flat" tlist generated by create_plan, though
+ * possibly in a different order. In that case we'll use create_plan's tlist,
+ * and the tlist made here is only needed as input to query_planner to tell
+ * it which Vars are needed in the output of the scan/join plan.
*
* 'tlist' is the query's target list.
* 'groupColIdx' receives an array of column numbers for the GROUP BY
- * expressions (if there are any) in the subplan's target list.
+ * expressions (if there are any) in the returned target list.
* 'need_tlist_eval' is set true if we really need to evaluate the
- * result tlist.
+ * returned tlist as-is.
*
- * The result is the targetlist to be passed to the subplan.
+ * The result is the targetlist to be passed to query_planner.
*/
static List *
make_subplanTargetList(PlannerInfo *root,
@@ -2649,7 +2653,7 @@ make_subplanTargetList(PlannerInfo *root,
/*
* locate_grouping_columns
- * Locate grouping columns in the tlist chosen by query_planner.
+ * Locate grouping columns in the tlist chosen by create_plan.
*
* This is only needed if we don't use the sub_tlist chosen by
* make_subplanTargetList. We have to forget the column indexes found
--
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