Code review for foreign/custom join pushdown patch.

Commit e7cb7ee14555cc9c5773e2c102efd6371f6f2005 included some design
decisions that seem pretty questionable to me, and there was quite a lot
of stuff not to like about the documentation and comments.  Clean up
as follows:

* Consider foreign joins only between foreign tables on the same server,
rather than between any two foreign tables with the same underlying FDW
handler function.  In most if not all cases, the FDW would simply have had
to apply the same-server restriction itself (far more expensively, both for
lack of caching and because it would be repeated for each combination of
input sub-joins), or else risk nasty bugs.  Anyone who's really intent on
doing something outside this restriction can always use the
set_join_pathlist_hook.

* Rename fdw_ps_tlist/custom_ps_tlist to fdw_scan_tlist/custom_scan_tlist
to better reflect what they're for, and allow these custom scan tlists
to be used even for base relations.

* Change make_foreignscan() API to include passing the fdw_scan_tlist
value, since the FDW is required to set that.  Backwards compatibility
doesn't seem like an adequate reason to expect FDWs to set it in some
ad-hoc extra step, and anyway existing FDWs can just pass NIL.

* Change the API of path-generating subroutines of add_paths_to_joinrel,
and in particular that of GetForeignJoinPaths and set_join_pathlist_hook,
so that various less-used parameters are passed in a struct rather than
as separate parameter-list entries.  The objective here is to reduce the
probability that future additions to those parameter lists will result in
source-level API breaks for users of these hooks.  It's possible that this
is even a small win for the core code, since most CPU architectures can't
pass more than half a dozen parameters efficiently anyway.  I kept root,
joinrel, outerrel, innerrel, and jointype as separate parameters to reduce
code churn in joinpath.c --- in particular, putting jointype into the
struct would have been problematic because of the subroutines' habit of
changing their local copies of that variable.

* Avoid ad-hocery in ExecAssignScanProjectionInfo.  It was probably all
right for it to know about IndexOnlyScan, but if the list is to grow
we should refactor the knowledge out to the callers.

* Restore nodeForeignscan.c's previous use of the relcache to avoid
extra GetFdwRoutine lookups for base-relation scans.

* Lots of cleanup of documentation and missed comments.  Re-order some
code additions into more logical places.

1 files changed, 25 insertions, 58 deletions

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 3246332d6e3..c8092372831 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -44,6 +44,7 @@
 #include "utils/lsyscache.h"
 
 
+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);
@@ -219,7 +220,7 @@ create_plan(PlannerInfo *root, Path *best_path)
  * create_plan_recurse
  *	  Recursive guts of create_plan().
  */
-Plan *
+static Plan *
 create_plan_recurse(PlannerInfo *root, Path *best_path)
 {
 	Plan	   *plan;
@@ -1950,7 +1951,7 @@ create_worktablescan_plan(PlannerInfo *root, Path *best_path,
 
 /*
  * create_foreignscan_plan
- *	 Returns a foreignscan plan for the base relation scanned by 'best_path'
+ *	 Returns a foreignscan plan for the relation scanned by 'best_path'
  *	 with restriction clauses 'scan_clauses' and targetlist 'tlist'.
  */
 static ForeignScan *
@@ -1965,9 +1966,11 @@ create_foreignscan_plan(PlannerInfo *root, ForeignPath *best_path,
 	ListCell   *lc;
 	int			i;
 
+	Assert(rel->fdwroutine != NULL);
+
 	/*
-	 * If we're scanning a base relation, look up the OID.
-	 * (We can skip this if scanning a join relation.)
+	 * If we're scanning a base relation, fetch its OID.  (Irrelevant if
+	 * scanning a join relation.)
 	 */
 	if (scan_relid > 0)
 	{
@@ -1978,7 +1981,6 @@ create_foreignscan_plan(PlannerInfo *root, ForeignPath *best_path,
 		Assert(rte->rtekind == RTE_RELATION);
 		rel_oid = rte->relid;
 	}
-	Assert(rel->fdwroutine != NULL);
 
 	/*
 	 * Sort clauses into best execution order.  We do this first since the FDW
@@ -1996,42 +1998,22 @@ create_foreignscan_plan(PlannerInfo *root, ForeignPath *best_path,
 	scan_plan = rel->fdwroutine->GetForeignPlan(root, rel, rel_oid,
 												best_path,
 												tlist, scan_clauses);
-	/*
-	 * Sanity check.  There may be resjunk entries in fdw_ps_tlist that
-	 * are included only to help EXPLAIN deparse plans properly. We require
-	 * that these are at the end, so that when the executor builds the scan
-	 * descriptor based on the non-junk entries, it gets the attribute
-	 * numbers correct.
-	 */
-	if (scan_plan->scan.scanrelid == 0)
-	{
-		bool	found_resjunk = false;
-
-		foreach (lc, scan_plan->fdw_ps_tlist)
-		{
-			TargetEntry	   *tle = lfirst(lc);
-
-			if (tle->resjunk)
-				found_resjunk = true;
-			else if (found_resjunk)
-				elog(ERROR, "junk TLE should not apper prior to valid one");
-		}
-	}
-	/* Set the relids that are represented by this foreign scan for Explain */
-	scan_plan->fdw_relids = best_path->path.parent->relids;
 
 	/* Copy cost data from Path to Plan; no need to make FDW do this */
 	copy_path_costsize(&scan_plan->scan.plan, &best_path->path);
 
-	/* Track FDW server-id; no need to make FDW do this */
-	scan_plan->fdw_handler = rel->fdw_handler;
+	/* Copy foreign server OID; likewise, no need to make FDW do this */
+	scan_plan->fs_server = rel->serverid;
+
+	/* Likewise, copy the relids that are represented by this foreign scan */
+	scan_plan->fs_relids = best_path->path.parent->relids;
 
 	/*
 	 * Replace any outer-relation variables with nestloop params in the qual
 	 * and fdw_exprs expressions.  We do this last so that the FDW doesn't
 	 * have to be involved.  (Note that parts of fdw_exprs could have come
 	 * from join clauses, so doing this beforehand on the scan_clauses
-	 * wouldn't work.)
+	 * wouldn't work.)  We assume fdw_scan_tlist contains no such variables.
 	 */
 	if (best_path->path.param_info)
 	{
@@ -2087,7 +2069,6 @@ create_customscan_plan(PlannerInfo *root, CustomPath *best_path,
 {
 	CustomScan *cplan;
 	RelOptInfo *rel = best_path->path.parent;
-	ListCell   *lc;
 
 	/*
 	 * Sort clauses into the best execution order, although custom-scan
@@ -2107,41 +2088,21 @@ create_customscan_plan(PlannerInfo *root, CustomPath *best_path,
 	Assert(IsA(cplan, CustomScan));
 
 	/*
-	 * Sanity check.  There may be resjunk entries in custom_ps_tlist that
-	 * are included only to help EXPLAIN deparse plans properly. We require
-	 * that these are at the end, so that when the executor builds the scan
-	 * descriptor based on the non-junk entries, it gets the attribute
-	 * numbers correct.
-	 */
-	if (cplan->scan.scanrelid == 0)
-	{
-		bool	found_resjunk = false;
-
-		foreach (lc, cplan->custom_ps_tlist)
-		{
-			TargetEntry	   *tle = lfirst(lc);
-
-			if (tle->resjunk)
-				found_resjunk = true;
-			else if (found_resjunk)
-				elog(ERROR, "junk TLE should not apper prior to valid one");
-		}
-	}
-	/* Set the relids that are represented by this custom scan for Explain */
-	cplan->custom_relids = best_path->path.parent->relids;
-
-	/*
 	 * Copy cost data from Path to Plan; no need to make custom-plan providers
 	 * do this
 	 */
 	copy_path_costsize(&cplan->scan.plan, &best_path->path);
 
+	/* Likewise, copy the relids that are represented by this custom scan */
+	cplan->custom_relids = best_path->path.parent->relids;
+
 	/*
 	 * Replace any outer-relation variables with nestloop params in the qual
 	 * and custom_exprs expressions.  We do this last so that the custom-plan
 	 * provider doesn't have to be involved.  (Note that parts of custom_exprs
 	 * could have come from join clauses, so doing this beforehand on the
-	 * scan_clauses wouldn't work.)
+	 * scan_clauses wouldn't work.)  We assume custom_scan_tlist contains no
+	 * such variables.
 	 */
 	if (best_path->path.param_info)
 	{
@@ -3611,7 +3572,8 @@ make_foreignscan(List *qptlist,
 				 List *qpqual,
 				 Index scanrelid,
 				 List *fdw_exprs,
-				 List *fdw_private)
+				 List *fdw_private,
+				 List *fdw_scan_tlist)
 {
 	ForeignScan *node = makeNode(ForeignScan);
 	Plan	   *plan = &node->scan.plan;
@@ -3622,8 +3584,13 @@ make_foreignscan(List *qptlist,
 	plan->lefttree = NULL;
 	plan->righttree = NULL;
 	node->scan.scanrelid = scanrelid;
+	/* fs_server will be filled in by create_foreignscan_plan */
+	node->fs_server = InvalidOid;
 	node->fdw_exprs = fdw_exprs;
 	node->fdw_private = fdw_private;
+	node->fdw_scan_tlist = fdw_scan_tlist;
+	/* fs_relids will be filled in by create_foreignscan_plan */
+	node->fs_relids = NULL;
 	/* fsSystemCol will be filled in by create_foreignscan_plan */
 	node->fsSystemCol = false;