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, 10 insertions, 9 deletions

diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 4b3cd85ad90..156b5331f36 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -128,8 +128,8 @@ typedef struct
  * varlevelsup > 0).  We store the PlanState node that is the immediate
  * parent of the expression to be deparsed, as well as a list of that
  * PlanState's ancestors.  In addition, we store its outer and inner subplan
- * state nodes, as well as their plan nodes' targetlists, and the indextlist
- * if the current PlanState is an IndexOnlyScanState.  (These fields could
+ * state nodes, as well as their plan nodes' targetlists, and the index tlist
+ * if the current plan node might contain INDEX_VAR Vars.  (These fields could
  * be derived on-the-fly from the current PlanState, but it seems notationally
  * clearer to set them up as separate fields.)
  */
@@ -2586,10 +2586,11 @@ deparse_context_for_plan_rtable(List *rtable, List *rtable_names)
  * provide the parent PlanState node.  Then OUTER_VAR and INNER_VAR references
  * can be resolved by drilling down into the left and right child plans.
  * Similarly, INDEX_VAR references can be resolved by reference to the
- * indextlist given in the parent IndexOnlyScan node.  (Note that we don't
- * currently support deparsing of indexquals in regular IndexScan or
- * BitmapIndexScan nodes; for those, we can only deparse the indexqualorig
- * fields, which won't contain INDEX_VAR Vars.)
+ * indextlist given in a parent IndexOnlyScan node, or to the scan tlist in
+ * ForeignScan and CustomScan nodes.  (Note that we don't currently support
+ * deparsing of indexquals in regular IndexScan or BitmapIndexScan nodes;
+ * for those, we can only deparse the indexqualorig fields, which won't
+ * contain INDEX_VAR Vars.)
  *
  * Note: planstate really ought to be declared as "PlanState *", but we use
  * "Node *" to avoid having to include execnodes.h in ruleutils.h.
@@ -3870,13 +3871,13 @@ set_deparse_planstate(deparse_namespace *dpns, PlanState *ps)
 	else
 		dpns->inner_tlist = NIL;
 
-	/* index_tlist is set only if it's an IndexOnlyScan */
+	/* Set up referent for INDEX_VAR Vars, if needed */
 	if (IsA(ps->plan, IndexOnlyScan))
 		dpns->index_tlist = ((IndexOnlyScan *) ps->plan)->indextlist;
 	else if (IsA(ps->plan, ForeignScan))
-		dpns->index_tlist = ((ForeignScan *) ps->plan)->fdw_ps_tlist;
+		dpns->index_tlist = ((ForeignScan *) ps->plan)->fdw_scan_tlist;
 	else if (IsA(ps->plan, CustomScan))
-		dpns->index_tlist = ((CustomScan *) ps->plan)->custom_ps_tlist;
+		dpns->index_tlist = ((CustomScan *) ps->plan)->custom_scan_tlist;
 	else
 		dpns->index_tlist = NIL;
 }