Avoid postgres_fdw crash for a targetlist entry that's just a Param.

foreign_grouping_ok() is willing to put fairly arbitrary expressions into
the targetlist of a remote SELECT that's doing grouping or aggregation on
the remote side, including expressions that have no foreign component to
them at all.  This is possibly a bit dubious from an efficiency standpoint;
but it rises to the level of a crash-causing bug if the expression is just
a Param or non-foreign Var.  In that case, the expression will necessarily
also appear in the fdw_exprs list of values we need to send to the remote
server, and then setrefs.c's set_foreignscan_references will mistakenly
replace the fdw_exprs entry with a Var referencing the targetlist result.

The root cause of this problem is bad design in commit e7cb7ee14: it put
logic into set_foreignscan_references that IMV is postgres_fdw-specific,
and yet this bug shows that it isn't postgres_fdw-specific enough.  The
transformation being done on fdw_exprs assumes that fdw_exprs is to be
evaluated with the fdw_scan_tlist as input, which is not how postgres_fdw
uses it; yet it could be the right thing for some other FDW.  (In the
bigger picture, setrefs.c has no business assuming this for the other
expression fields of a ForeignScan either.)

The right fix therefore would be to expand the FDW API so that the
FDW could inform setrefs.c how it intends to evaluate these various
expressions.  We can't change that in the back branches though, and we
also can't just summarily change setrefs.c's behavior there, or we're
likely to break external FDWs.

As a stopgap, therefore, hack up postgres_fdw so that it won't attempt
to send targetlist entries that look exactly like the fdw_exprs entries
they'd produce.  In most cases this actually produces a superior plan,
IMO, with less data needing to be transmitted and returned; so we probably
ought to think harder about whether we should ship tlist expressions at
all when they don't contain any foreign Vars or Aggs.  But that's an
optimization not a bug fix so I left it for later.  One case where this
produces an inferior plan is where the expression in question is actually
a GROUP BY expression: then the restriction prevents us from using remote
grouping.  It might be possible to work around that (since that would
reduce to group-by-a-constant on the remote side); but it seems like a
pretty unlikely corner case, so I'm not sure it's worth expending effort
solely to improve that.  In any case the right long-term answer is to fix
the API as sketched above, and then revert this hack.

Per bug #15781 from Sean Johnston.  Back-patch to v10 where the problem
was introduced.

Discussion: https://postgr.es/m/15781-2601b1002bad087c@postgresql.org

1 files changed, 27 insertions, 5 deletions

diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 794363c184c..6c9e320af99 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -5486,7 +5486,6 @@ foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel,
 	PgFdwRelationInfo *fpinfo = (PgFdwRelationInfo *) grouped_rel->fdw_private;
 	PathTarget *grouping_target = grouped_rel->reltarget;
 	PgFdwRelationInfo *ofpinfo;
-	List	   *aggvars;
 	ListCell   *lc;
 	int			i;
 	List	   *tlist = NIL;
@@ -5512,6 +5511,15 @@ foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel,
 	 * server.  All GROUP BY expressions will be part of the grouping target
 	 * and thus there is no need to search for them separately.  Add grouping
 	 * expressions into target list which will be passed to foreign server.
+	 *
+	 * A tricky fine point is that we must not put any expression into the
+	 * target list that is just a foreign param (that is, something that
+	 * deparse.c would conclude has to be sent to the foreign server).  If we
+	 * do, the expression will also appear in the fdw_exprs list of the plan
+	 * node, and setrefs.c will get confused and decide that the fdw_exprs
+	 * entry is actually a reference to the fdw_scan_tlist entry, resulting in
+	 * a broken plan.  Somewhat oddly, it's OK if the expression contains such
+	 * a node, as long as it's not at top level; then no match is possible.
 	 */
 	i = 0;
 	foreach(lc, grouping_target->exprs)
@@ -5533,6 +5541,13 @@ foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel,
 				return false;
 
 			/*
+			 * If it would be a foreign param, we can't put it into the tlist,
+			 * so we have to fail.
+			 */
+			if (is_foreign_param(root, grouped_rel, expr))
+				return false;
+
+			/*
 			 * Pushable, so add to tlist.  We need to create a TLE for this
 			 * expression and apply the sortgroupref to it.  We cannot use
 			 * add_to_flat_tlist() here because that avoids making duplicate
@@ -5547,9 +5562,11 @@ foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel,
 		else
 		{
 			/*
-			 * Non-grouping expression we need to compute.  Is it shippable?
+			 * Non-grouping expression we need to compute.  Can we ship it
+			 * as-is to the foreign server?
 			 */
-			if (is_foreign_expr(root, grouped_rel, expr))
+			if (is_foreign_expr(root, grouped_rel, expr) &&
+				!is_foreign_param(root, grouped_rel, expr))
 			{
 				/* Yes, so add to tlist as-is; OK to suppress duplicates */
 				tlist = add_to_flat_tlist(tlist, list_make1(expr));
@@ -5557,12 +5574,16 @@ foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel,
 			else
 			{
 				/* Not pushable as a whole; extract its Vars and aggregates */
+				List	   *aggvars;
+
 				aggvars = pull_var_clause((Node *) expr,
 										  PVC_INCLUDE_AGGREGATES);
 
 				/*
 				 * If any aggregate expression is not shippable, then we
-				 * cannot push down aggregation to the foreign server.
+				 * cannot push down aggregation to the foreign server.  (We
+				 * don't have to check is_foreign_param, since that certainly
+				 * won't return true for any such expression.)
 				 */
 				if (!is_foreign_expr(root, grouped_rel, (Expr *) aggvars))
 					return false;
@@ -5649,7 +5670,8 @@ foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel,
 			 * If aggregates within local conditions are not safe to push
 			 * down, then we cannot push down the query.  Vars are already
 			 * part of GROUP BY clause which are checked above, so no need to
-			 * access them again here.
+			 * access them again here.  Again, we need not check
+			 * is_foreign_param for a foreign aggregate.
 			 */
 			if (IsA(expr, Aggref))
 			{