Always build a custom plan node's targetlist from the path's pathtarget.

We were applying the use_physical_tlist optimization to all relation
scan plans, even those implemented by custom scan providers.  However,
that's a bad idea for a couple of reasons.  The custom provider might
be unable to provide columns that it hadn't expected to be asked for
(for example, the custom scan might depend on an index-only scan).
Even more to the point, there's no good reason to suppose that this
"optimization" is a win for a custom scan; whatever the custom provider
is doing is likely not based on simply returning physical heap tuples.
(As a counterexample, if the custom scan is an interface to a column store,
demanding all columns would be a huge loss.)  If it is a win, the custom
provider could make that decision for itself and insert a suitable
pathtarget into the path, anyway.

Per discussion with Dmitry Ivanov.  Back-patch to 9.5 where custom scan
support was introduced.  The argument that the custom provider can adjust
the behavior by changing the pathtarget only applies to 9.6+, but on
balance it seems more likely that use_physical_tlist will hurt custom
scans than help them.

Discussion: https://postgr.es/m/e29ddd30-8ef9-4da5-a50b-2bb7b8c7198d@postgrespro.ru

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 6af68b08d27e133d2371dcf029b864cd11fdc46f..c464d054c942c8853c00c98026d8d3e6ebfbd38c 100644 (file)
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -47,7 +47,7 @@
 static Plan *create_plan_recurse(PlannerInfo *root, Path *best_path);
 static Plan *create_scan_plan(PlannerInfo *root, Path *best_path);
 static List *build_path_tlist(PlannerInfo *root, Path *path);
-static bool use_physical_tlist(PlannerInfo *root, RelOptInfo *rel);
+static bool use_physical_tlist(PlannerInfo *root, Path *path);
 static void disuse_physical_tlist(PlannerInfo *root, Plan *plan, Path *path);
 static Plan *create_gating_plan(PlannerInfo *root, Plan *plan, List *quals);
 static Plan *create_join_plan(PlannerInfo *root, JoinPath *best_path);
@@ -303,7 +303,7 @@ create_scan_plan(PlannerInfo *root, Path *best_path)
     * planner.c may replace the tlist we generate here, forcing projection to
     * occur.)
     */
-   if (use_physical_tlist(root, rel))
+   if (use_physical_tlist(root, best_path))
    {
        if (best_path->pathtype == T_IndexOnlyScan)
        {
@@ -493,8 +493,9 @@ build_path_tlist(PlannerInfo *root, Path *path)
  *     rather than only those Vars actually referenced.
  */
 static bool
-use_physical_tlist(PlannerInfo *root, RelOptInfo *rel)
+use_physical_tlist(PlannerInfo *root, Path *path)
 {
+   RelOptInfo *rel = path->parent;
    int         i;
    ListCell   *lc;
 
@@ -516,6 +517,13 @@ use_physical_tlist(PlannerInfo *root, RelOptInfo *rel)
    if (rel->reloptkind != RELOPT_BASEREL)
        return false;
 
+   /*
+    * Also, don't do it to a CustomPath; the premise that we're extracting
+    * columns from a simple physical tuple is unlikely to hold for those.
+    */
+   if (IsA(path, CustomPath))
+       return false;
+
    /*
     * Can't do it if any system columns or whole-row Vars are requested.
     * (This could possibly be fixed but would take some fragile assumptions