postgres_fdw: Perform the (FINAL, NULL) upperrel operations remotely.

The upper-planner pathification allows FDWs to arrange to push down
different types of upper-stage operations to the remote side.  This
commit teaches postgres_fdw to do it for the (FINAL, NULL) upperrel,
which is responsible for doing LockRows, LIMIT, and/or ModifyTable.
This provides the ability for postgres_fdw to handle SELECT commands
so that it 1) skips the LockRows step (if any) (note that this is
safe since it performs early locking) and 2) pushes down the LIMIT
and/or OFFSET restrictions (if any) to the remote side.  This doesn't
handle the INSERT/UPDATE/DELETE cases.

Author: Etsuro Fujita
Reviewed-By: Antonin Houska and Jeff Janes
Discussion: https://postgr.es/m/87pnz1aby9.fsf@news-spur.riddles.org.uk

Author: Etsuro Fujita

contrib/postgres_fdw/deparse.c

@@ -169,6 +169,7 @@ static void deparseSelectSql(List *tlist, bool is_subquery, List **retrieved_att
 static void deparseLockingClause(deparse_expr_cxt *context);
 static void appendOrderByClause(List *pathkeys, bool has_final_sort,
 					deparse_expr_cxt *context);
+static void appendLimitClause(deparse_expr_cxt *context);
 static void appendConditions(List *exprs, deparse_expr_cxt *context);
 static void deparseFromExprForRel(StringInfo buf, PlannerInfo *root,
 					  RelOptInfo *foreignrel, bool use_alias,
@@ -930,7 +931,7 @@ build_tlist_to_deparse(RelOptInfo *foreignrel)
 void
 deparseSelectStmtForRel(StringInfo buf, PlannerInfo *root, RelOptInfo *rel,
 						List *tlist, List *remote_conds, List *pathkeys,
-						bool has_final_sort, bool is_subquery,
+						bool has_final_sort, bool has_limit, bool is_subquery,
 						List **retrieved_attrs, List **params_list)
 {
 	deparse_expr_cxt context;
@@ -988,6 +989,10 @@ deparseSelectStmtForRel(StringInfo buf, PlannerInfo *root, RelOptInfo *rel,
 	if (pathkeys)
 		appendOrderByClause(pathkeys, has_final_sort, &context);
 
+	/* Add LIMIT clause if necessary */
+	if (has_limit)
+		appendLimitClause(&context);
+
 	/* Add any necessary FOR UPDATE/SHARE. */
 	deparseLockingClause(&context);
 }
@@ -1591,7 +1596,8 @@ deparseRangeTblRef(StringInfo buf, PlannerInfo *root, RelOptInfo *foreignrel,
 		/* Deparse the subquery representing the relation. */
 		appendStringInfoChar(buf, '(');
 		deparseSelectStmtForRel(buf, root, foreignrel, NIL,
-								fpinfo->remote_conds, NIL, false, true,
+								fpinfo->remote_conds, NIL,
+								false, false, true,
 								&retrieved_attrs, params_list);
 		appendStringInfoChar(buf, ')');
 
@@ -3160,6 +3166,33 @@ appendOrderByClause(List *pathkeys, bool has_final_sort,
 	reset_transmission_modes(nestlevel);
 }
 
+/*
+ * Deparse LIMIT/OFFSET clause.
+ */
+static void
+appendLimitClause(deparse_expr_cxt *context)
+{
+	PlannerInfo *root = context->root;
+	StringInfo	buf = context->buf;
+	int			nestlevel;
+
+	/* Make sure any constants in the exprs are printed portably */
+	nestlevel = set_transmission_modes();
+
+	if (root->parse->limitCount)
+	{
+		appendStringInfoString(buf, " LIMIT ");
+		deparseExpr((Expr *) root->parse->limitCount, context);
+	}
+	if (root->parse->limitOffset)
+	{
+		appendStringInfoString(buf, " OFFSET ");
+		deparseExpr((Expr *) root->parse->limitOffset, context);
+	}
+
+	reset_transmission_modes(nestlevel);
+}
+
 /*
  * appendFunctionName
  *		Deparses function name from given function oid.