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, 38 insertions, 17 deletions

diff --git a/src/backend/executor/nodeCustom.c b/src/backend/executor/nodeCustom.c
index db1b4f2ffa4..0a022dff940 100644
--- a/src/backend/executor/nodeCustom.c
+++ b/src/backend/executor/nodeCustom.c
@@ -22,13 +22,24 @@
 CustomScanState *
 ExecInitCustomScan(CustomScan *cscan, EState *estate, int eflags)
 {
-	CustomScanState    *css;
-	Index				scan_relid = cscan->scan.scanrelid;
+	CustomScanState *css;
+	Relation	scan_rel = NULL;
+	Index		scanrelid = cscan->scan.scanrelid;
+	Index		tlistvarno;
 
-	/* populate a CustomScanState according to the CustomScan */
+	/*
+	 * Allocate the CustomScanState object.  We let the custom scan provider
+	 * do the palloc, in case it wants to make a larger object that embeds
+	 * CustomScanState as the first field.  It must set the node tag and the
+	 * methods field correctly at this time.  Other standard fields should be
+	 * set to zero.
+	 */
 	css = (CustomScanState *) cscan->methods->CreateCustomScanState(cscan);
 	Assert(IsA(css, CustomScanState));
 
+	/* ensure flags is filled correctly */
+	css->flags = cscan->flags;
+
 	/* fill up fields of ScanState */
 	css->ss.ps.plan = &cscan->scan.plan;
 	css->ss.ps.state = estate;
@@ -36,6 +47,8 @@ ExecInitCustomScan(CustomScan *cscan, EState *estate, int eflags)
 	/* create expression context for node */
 	ExecAssignExprContext(estate, &css->ss.ps);
 
+	css->ss.ps.ps_TupFromTlist = false;
+
 	/* initialize child expressions */
 	css->ss.ps.targetlist = (List *)
 		ExecInitExpr((Expr *) cscan->scan.plan.targetlist,
@@ -49,32 +62,40 @@ ExecInitCustomScan(CustomScan *cscan, EState *estate, int eflags)
 	ExecInitResultTupleSlot(estate, &css->ss.ps);
 
 	/*
-	 * open the base relation and acquire an appropriate lock on it;
-	 * also, get and assign the scan type
+	 * open the base relation, if any, and acquire an appropriate lock on it
 	 */
-	if (scan_relid > 0)
+	if (scanrelid > 0)
 	{
-		Relation		scan_rel;
-
-		scan_rel = ExecOpenScanRelation(estate, scan_relid, eflags);
+		scan_rel = ExecOpenScanRelation(estate, scanrelid, eflags);
 		css->ss.ss_currentRelation = scan_rel;
-		css->ss.ss_currentScanDesc = NULL;	/* set by provider */
-		ExecAssignScanType(&css->ss, RelationGetDescr(scan_rel));
 	}
-	else
+
+	/*
+	 * Determine the scan tuple type.  If the custom scan provider provided a
+	 * targetlist describing the scan tuples, use that; else use base
+	 * relation's rowtype.
+	 */
+	if (cscan->custom_scan_tlist != NIL || scan_rel == NULL)
 	{
-		TupleDesc	ps_tupdesc;
+		TupleDesc	scan_tupdesc;
 
-		ps_tupdesc = ExecCleanTypeFromTL(cscan->custom_ps_tlist, false);
-		ExecAssignScanType(&css->ss, ps_tupdesc);
+		scan_tupdesc = ExecTypeFromTL(cscan->custom_scan_tlist, false);
+		ExecAssignScanType(&css->ss, scan_tupdesc);
+		/* Node's targetlist will contain Vars with varno = INDEX_VAR */
+		tlistvarno = INDEX_VAR;
+	}
+	else
+	{
+		ExecAssignScanType(&css->ss, RelationGetDescr(scan_rel));
+		/* Node's targetlist will contain Vars with varno = scanrelid */
+		tlistvarno = scanrelid;
 	}
-	css->ss.ps.ps_TupFromTlist = false;
 
 	/*
 	 * Initialize result tuple type and projection info.
 	 */
 	ExecAssignResultTypeFromTL(&css->ss.ps);
-	ExecAssignScanProjectionInfo(&css->ss);
+	ExecAssignScanProjectionInfoWithVarno(&css->ss, tlistvarno);
 
 	/*
 	 * The callback of custom-scan provider applies the final initialization