Allow FDWs to push down quals without breaking EvalPlanQual rechecks.

This fixes a long-standing bug which was discovered while investigating
the interaction between the new join pushdown code and the EvalPlanQual
machinery: if a ForeignScan appears on the inner side of a paramaterized
nestloop, an EPQ recheck would re-return the original tuple even if
it no longer satisfied the pushed-down quals due to changed parameter
values.

This fix adds a new member to ForeignScan and ForeignScanState and a
new argument to make_foreignscan, and requires changes to FDWs which
push down quals to populate that new argument with a list of quals they
have chosen to push down.  Therefore, I'm only back-patching to 9.5,
even though the bug is not new in 9.5.

Etsuro Fujita, reviewed by me and by Kyotaro Horiguchi.

Author: robertmhaas

contrib/file_fdw/file_fdw.c

@@ -563,7 +563,8 @@ fileGetForeignPlan(PlannerInfo *root,
 							scan_relid,
 							NIL,	/* no expressions to evaluate */
 							best_path->fdw_private,
-							NIL /* no custom tlist */ );
+							NIL,	/* no custom tlist */
+							NIL /* no remote quals */ );
 }
 
 /*

contrib/postgres_fdw/postgres_fdw.c

@@ -748,6 +748,7 @@ postgresGetForeignPlan(PlannerInfo *root,
 	Index		scan_relid = baserel->relid;
 	List	   *fdw_private;
 	List	   *remote_conds = NIL;
+	List	   *remote_exprs = NIL;
 	List	   *local_exprs = NIL;
 	List	   *params_list = NIL;
 	List	   *retrieved_attrs;
@@ -769,8 +770,8 @@ postgresGetForeignPlan(PlannerInfo *root,
 	 *
 	 * This code must match "extract_actual_clauses(scan_clauses, false)"
 	 * except for the additional decision about remote versus local execution.
-	 * Note however that we only strip the RestrictInfo nodes from the
-	 * local_exprs list, since appendWhereClause expects a list of
+	 * Note however that we don't strip the RestrictInfo nodes from the
+	 * remote_conds list, since appendWhereClause expects a list of
 	 * RestrictInfos.
 	 */
 	foreach(lc, scan_clauses)
@@ -784,11 +785,17 @@ postgresGetForeignPlan(PlannerInfo *root,
 			continue;
 
 		if (list_member_ptr(fpinfo->remote_conds, rinfo))
+		{
 			remote_conds = lappend(remote_conds, rinfo);
+			remote_exprs = lappend(remote_exprs, rinfo->clause);
+		}
 		else if (list_member_ptr(fpinfo->local_conds, rinfo))
 			local_exprs = lappend(local_exprs, rinfo->clause);
 		else if (is_foreign_expr(root, baserel, rinfo->clause))
+		{
 			remote_conds = lappend(remote_conds, rinfo);
+			remote_exprs = lappend(remote_exprs, rinfo->clause);
+		}
 		else
 			local_exprs = lappend(local_exprs, rinfo->clause);
 	}
@@ -874,7 +881,8 @@ postgresGetForeignPlan(PlannerInfo *root,
 							scan_relid,
 							params_list,
 							fdw_private,
-							NIL /* no custom tlist */ );
+							NIL,	/* no custom tlist */
+							remote_exprs);
 }
 
 /*

doc/src/sgml/fdwhandler.sgml

@@ -1135,6 +1135,15 @@ GetForeignServerByName(const char *name, bool missing_ok);
      evaluation of the <structfield>fdw_exprs</> expression tree.
     </para>
 
+    <para>
+     Any clauses removed from the plan node's qual list must instead be added
+     to <literal>fdw_recheck_quals</> in order to ensure correct behavior
+     at the <literal>READ COMMITTED</> isolation level.  When a concurrent
+     update occurs for some other table involved in the query, the executor
+     may need to verify that all of the original quals are still satisfied for
+     the tuple, possibly against a different set of parameter values.
+    </para>
+
     <para>
      Another <structname>ForeignScan</> field that can be filled by FDWs
      is <structfield>fdw_scan_tlist</>, which describes the tuples returned by

src/backend/executor/nodeForeignscan.c

@@ -25,6 +25,7 @@
 #include "executor/executor.h"
 #include "executor/nodeForeignscan.h"
 #include "foreign/fdwapi.h"
+#include "utils/memutils.h"
 #include "utils/rel.h"
 
 static TupleTableSlot *ForeignNext(ForeignScanState *node);
@@ -72,8 +73,19 @@ ForeignNext(ForeignScanState *node)
 static bool
 ForeignRecheck(ForeignScanState *node, TupleTableSlot *slot)
 {
-	/* There are no access-method-specific conditions to recheck. */
-	return true;
+	ExprContext *econtext;
+
+	/*
+	 * extract necessary information from foreign scan node
+	 */
+	econtext = node->ss.ps.ps_ExprContext;
+
+	/* Does the tuple meet the remote qual condition? */
+	econtext->ecxt_scantuple = slot;
+
+	ResetExprContext(econtext);
+
+	return ExecQual(node->fdw_recheck_quals, econtext, false);
 }
 
 /* ----------------------------------------------------------------
@@ -135,6 +147,9 @@ ExecInitForeignScan(ForeignScan *node, EState *estate, int eflags)
 	scanstate->ss.ps.qual = (List *)
 		ExecInitExpr((Expr *) node->scan.plan.qual,
 					 (PlanState *) scanstate);
+	scanstate->fdw_recheck_quals = (List *)
+		ExecInitExpr((Expr *) node->fdw_recheck_quals,
+					 (PlanState *) scanstate);
 
 	/*
 	 * tuple table initialization

src/backend/nodes/copyfuncs.c

@@ -648,6 +648,7 @@ _copyForeignScan(const ForeignScan *from)
 	COPY_NODE_FIELD(fdw_exprs);
 	COPY_NODE_FIELD(fdw_private);
 	COPY_NODE_FIELD(fdw_scan_tlist);
+	COPY_NODE_FIELD(fdw_recheck_quals);
 	COPY_BITMAPSET_FIELD(fs_relids);
 	COPY_SCALAR_FIELD(fsSystemCol);
 

src/backend/nodes/outfuncs.c

@@ -594,6 +594,7 @@ _outForeignScan(StringInfo str, const ForeignScan *node)
 	WRITE_NODE_FIELD(fdw_exprs);
 	WRITE_NODE_FIELD(fdw_private);
 	WRITE_NODE_FIELD(fdw_scan_tlist);
+	WRITE_NODE_FIELD(fdw_recheck_quals);
 	WRITE_BITMAPSET_FIELD(fs_relids);
 	WRITE_BOOL_FIELD(fsSystemCol);
 }

src/backend/nodes/readfuncs.c

@@ -1798,6 +1798,7 @@ _readForeignScan(void)
 	READ_NODE_FIELD(fdw_exprs);
 	READ_NODE_FIELD(fdw_private);
 	READ_NODE_FIELD(fdw_scan_tlist);
+	READ_NODE_FIELD(fdw_recheck_quals);
 	READ_BITMAPSET_FIELD(fs_relids);
 	READ_BOOL_FIELD(fsSystemCol);
 

src/backend/optimizer/plan/createplan.c

@@ -2153,6 +2153,9 @@ create_foreignscan_plan(PlannerInfo *root, ForeignPath *best_path,
 			replace_nestloop_params(root, (Node *) scan_plan->scan.plan.qual);
 		scan_plan->fdw_exprs = (List *)
 			replace_nestloop_params(root, (Node *) scan_plan->fdw_exprs);
+		scan_plan->fdw_recheck_quals = (List *)
+			replace_nestloop_params(root,
+									(Node *) scan_plan->fdw_recheck_quals);
 	}
 
 	/*
@@ -3738,7 +3741,8 @@ make_foreignscan(List *qptlist,
 				 Index scanrelid,
 				 List *fdw_exprs,
 				 List *fdw_private,
-				 List *fdw_scan_tlist)
+				 List *fdw_scan_tlist,
+				 List *fdw_recheck_quals)
 {
 	ForeignScan *node = makeNode(ForeignScan);
 	Plan	   *plan = &node->scan.plan;
@@ -3754,6 +3758,7 @@ make_foreignscan(List *qptlist,
 	node->fdw_exprs = fdw_exprs;
 	node->fdw_private = fdw_private;
 	node->fdw_scan_tlist = fdw_scan_tlist;
+	node->fdw_recheck_quals = fdw_recheck_quals;
 	/* fs_relids will be filled in by create_foreignscan_plan */
 	node->fs_relids = NULL;
 	/* fsSystemCol will be filled in by create_foreignscan_plan */

src/backend/optimizer/plan/setrefs.c

@@ -1133,13 +1133,15 @@ set_foreignscan_references(PlannerInfo *root,
 	}
 	else
 	{
-		/* Adjust tlist, qual, fdw_exprs in the standard way */
+		/* Adjust tlist, qual, fdw_exprs, etc. in the standard way */
 		fscan->scan.plan.targetlist =
 			fix_scan_list(root, fscan->scan.plan.targetlist, rtoffset);
 		fscan->scan.plan.qual =
 			fix_scan_list(root, fscan->scan.plan.qual, rtoffset);
 		fscan->fdw_exprs =
 			fix_scan_list(root, fscan->fdw_exprs, rtoffset);
+		fscan->fdw_recheck_quals =
+			fix_scan_list(root, fscan->fdw_recheck_quals, rtoffset);
 	}
 
 	/* Adjust fs_relids if needed */

src/backend/optimizer/plan/subselect.c

@@ -2394,10 +2394,18 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 			break;
 
 		case T_ForeignScan:
-			finalize_primnode((Node *) ((ForeignScan *) plan)->fdw_exprs,
-							  &context);
-			/* We assume fdw_scan_tlist cannot contain Params */
-			context.paramids = bms_add_members(context.paramids, scan_params);
+			{
+				ForeignScan *fscan = (ForeignScan *) plan;
+
+				finalize_primnode((Node *) fscan->fdw_exprs,
+								  &context);
+				finalize_primnode((Node *) fscan->fdw_recheck_quals,
+								  &context);
+
+				/* We assume fdw_scan_tlist cannot contain Params */
+				context.paramids = bms_add_members(context.paramids,
+												   scan_params);
+			}
 			break;
 
 		case T_CustomScan:

src/include/nodes/execnodes.h

@@ -1579,6 +1579,7 @@ typedef struct WorkTableScanState
 typedef struct ForeignScanState
 {
 	ScanState	ss;				/* its first field is NodeTag */
+	List	   *fdw_recheck_quals;	/* original quals not in ss.ps.qual */
 	/* use struct pointer to avoid including fdwapi.h here */
 	struct FdwRoutine *fdwroutine;
 	void	   *fdw_state;		/* foreign-data wrapper can keep state here */

src/include/nodes/plannodes.h

@@ -512,6 +512,11 @@ typedef struct WorkTableScan
  * fdw_scan_tlist is never actually executed; it just holds expression trees
  * describing what is in the scan tuple's columns.
  *
+ * fdw_recheck_quals should contain any quals which the core system passed to
+ * the FDW but which were not added to scan.plan.quals; that is, it should
+ * contain the quals being checked remotely.  This is needed for correct
+ * behavior during EvalPlanQual rechecks.
+ *
  * When the plan node represents a foreign join, scan.scanrelid is zero and
  * fs_relids must be consulted to identify the join relation.  (fs_relids
  * is valid for simple scans as well, but will always match scan.scanrelid.)
@@ -524,6 +529,7 @@ typedef struct ForeignScan
 	List	   *fdw_exprs;		/* expressions that FDW may evaluate */
 	List	   *fdw_private;	/* private data for FDW */
 	List	   *fdw_scan_tlist; /* optional tlist describing scan tuple */
+	List	   *fdw_recheck_quals;	/* original quals not in scan.plan.quals */
 	Bitmapset  *fs_relids;		/* RTIs generated by this scan */
 	bool		fsSystemCol;	/* true if any "system column" is needed */
 } ForeignScan;

src/include/optimizer/planmain.h

@@ -45,7 +45,7 @@ extern SubqueryScan *make_subqueryscan(List *qptlist, List *qpqual,
 				  Index scanrelid, Plan *subplan);
 extern ForeignScan *make_foreignscan(List *qptlist, List *qpqual,
 				 Index scanrelid, List *fdw_exprs, List *fdw_private,
-				 List *fdw_scan_tlist);
+				 List *fdw_scan_tlist, List *fdw_recheck_quals);
 extern Append *make_append(List *appendplans, List *tlist);
 extern RecursiveUnion *make_recursive_union(List *tlist,
 					 Plan *lefttree, Plan *righttree, int wtParam,