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.

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 98b673aca86e54abe925dd277ff8a14f83b9d88e..4ddb1211bb04d5e1512cdce52c6376d29ffe9c87 100644 (file)
--- 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