Skip unnecessary get_relids_in_jointree() when there are no PHVs

Commit 1df9e8d96 made remove_useless_result_rtes() compute the set of
baserels in the jointree, to pass down to the find_dependent_phvs()
checks.  But those checks are no-ops when the query contains no PHVs,
since find_dependent_phvs() and find_dependent_phvs_in_jointree() both
return early in that case.  So we can avoid the
get_relids_in_jointree() scan altogether when root->glob->lastPHId is
zero, leaving baserels as NULL.

Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAMbWs49H275KzgZr3Cd1Hy+6Lmwp35bZ+5PrVc62k3HDLj6hNQ@mail.gmail.com
Backpatch-through: 16

diff --git a/src/backend/optimizer/prep/prepjointree.c b/src/backend/optimizer/prep/prepjointree.c
index 1ea72af7c73b55f9ee87129e80c5bbee2397dff1..72d39ba30fd5ded6f9f955f586b60fc910aafd71 100644 (file)
--- a/src/backend/optimizer/prep/prepjointree.c
+++ b/src/backend/optimizer/prep/prepjointree.c
@@ -3888,16 +3888,18 @@ has_notnull_forced_var(PlannerInfo *root, List *forced_null_vars,
 void
 remove_useless_result_rtes(PlannerInfo *root)
 {
-       Relids          baserels;
+       Relids          baserels = NULL;
        Relids          dropped_outer_joins = NULL;
        ListCell   *cell;
 
        /*
         * We'll need the set of baserels in the jointree to perform
-        * find_dependent_phvs() checks.
+        * find_dependent_phvs() checks.  But if there are no PHVs anywhere in the
+        * query, those checks are no-ops, so we can skip the work.
         */
-       baserels = get_relids_in_jointree((Node *) root->parse->jointree,
-                                                                         false, false);
+       if (root->glob->lastPHId != 0)
+               baserels = get_relids_in_jointree((Node *) root->parse->jointree,
+                                                                                 false, false);
 
        /* Top level of jointree must always be a FromExpr */
        Assert(IsA(root->parse->jointree, FromExpr));
@@ -3967,7 +3969,8 @@ remove_useless_result_rtes(PlannerInfo *root)
  * (Note that in some cases, parent_quals points to the quals of a parent
  * more than one level up in the tree.)
  *
- * baserels is the set of base (non-join) RT indexes in the whole jointree.
+ * baserels is the set of base (non-join) RT indexes in the whole jointree;
+ * it can be NULL if the query contains no PHVs.
  */
 static Node *
 remove_useless_results_recurse(PlannerInfo *root, Node *jtnode,
@@ -4308,7 +4311,9 @@ remove_result_refs(PlannerInfo *root, int varno, Node *newjtloc)
  * already decided to remove those joins in remove_useless_result_rtes
  * and not yet have cleaned their relid bits out of upper PHVs.
  * But in general, it's the set of baserels that identify possible places
- * to evaluate a PHV, and we mustn't let that go to empty.
+ * to evaluate a PHV, and we mustn't let that go to empty.  (The caller is
+ * allowed to pass baserels as NULL if the query contains no PHVs at all,
+ * since then there is no work to do anyway.)
  *
  * find_dependent_phvs should be used when we want to see if there are
  * any such PHVs anywhere in the Query.  Another use-case is to see if
@@ -4320,7 +4325,7 @@ remove_result_refs(PlannerInfo *root, int varno, Node *newjtloc)
 typedef struct
 {
        Relids          relids;                 /* target relid, represented as a relid set */
-       Relids          baserels;               /* set of base (non-OJ) RT indexes in query */
+       Relids          baserels;               /* base RT indexes in query, NULL if no PHVs */
        int                     sublevels_up;   /* current nesting level */
 } find_dependent_phvs_context;