postgres_fdw: Refactor deparsing code for locking clauses.

The upcoming patch to allow join pushdown in postgres_fdw needs to use
this code multiple times, which requires moving it to deparse.c.  That
seems like a good idea anyway, so do that now both on general principle
and to simplify the future patch.

Inspired by a patch by Shigeru Hanada and Ashutosh Bapat, but I did
it a little differently than what that patch did.

Author: robertmhaas

contrib/postgres_fdw/deparse.c

@@ -45,7 +45,9 @@
 #include "catalog/pg_type.h"
 #include "commands/defrem.h"
 #include "nodes/nodeFuncs.h"
+#include "nodes/plannodes.h"
 #include "optimizer/clauses.h"
+#include "optimizer/prep.h"
 #include "optimizer/var.h"
 #include "parser/parsetree.h"
 #include "utils/builtins.h"
@@ -807,6 +809,64 @@ deparseTargetList(StringInfo buf,
 		appendStringInfoString(buf, "NULL");
 }
 
+/*
+ * Deparse the appropriate locking clause (FOR SELECT or FOR SHARE) for a
+ * given relation.
+ */
+void
+deparseLockingClause(StringInfo buf, PlannerInfo *root, RelOptInfo *rel)
+{
+	/*
+	 * Add FOR UPDATE/SHARE if appropriate.  We apply locking during the
+	 * initial row fetch, rather than later on as is done for local tables.
+	 * The extra roundtrips involved in trying to duplicate the local
+	 * semantics exactly don't seem worthwhile (see also comments for
+	 * RowMarkType).
+	 *
+	 * Note: because we actually run the query as a cursor, this assumes that
+	 * DECLARE CURSOR ... FOR UPDATE is supported, which it isn't before 8.3.
+	 */
+	if (rel->relid == root->parse->resultRelation &&
+		(root->parse->commandType == CMD_UPDATE ||
+		 root->parse->commandType == CMD_DELETE))
+	{
+		/* Relation is UPDATE/DELETE target, so use FOR UPDATE */
+		appendStringInfoString(buf, " FOR UPDATE");
+	}
+	else
+	{
+		PlanRowMark *rc = get_plan_rowmark(root->rowMarks, rel->relid);
+
+		if (rc)
+		{
+			/*
+			 * Relation is specified as a FOR UPDATE/SHARE target, so handle
+			 * that.  (But we could also see LCS_NONE, meaning this isn't a
+			 * target relation after all.)
+			 *
+			 * For now, just ignore any [NO] KEY specification, since (a) it's
+			 * not clear what that means for a remote table that we don't have
+			 * complete information about, and (b) it wouldn't work anyway on
+			 * older remote servers.  Likewise, we don't worry about NOWAIT.
+			 */
+			switch (rc->strength)
+			{
+				case LCS_NONE:
+					/* No locking needed */
+					break;
+				case LCS_FORKEYSHARE:
+				case LCS_FORSHARE:
+					appendStringInfoString(buf, " FOR SHARE");
+					break;
+				case LCS_FORNOKEYUPDATE:
+				case LCS_FORUPDATE:
+					appendStringInfoString(buf, " FOR UPDATE");
+					break;
+			}
+		}
+	}
+}
+
 /*
  * Deparse WHERE clauses in given list of RestrictInfos and append them to buf.
  *

contrib/postgres_fdw/postgres_fdw.c

@@ -1013,55 +1013,8 @@ postgresGetForeignPlan(PlannerInfo *root,
 	if (best_path->path.pathkeys)
 		appendOrderByClause(&sql, root, baserel, best_path->path.pathkeys);
 
-	/*
-	 * Add FOR UPDATE/SHARE if appropriate.  We apply locking during the
-	 * initial row fetch, rather than later on as is done for local tables.
-	 * The extra roundtrips involved in trying to duplicate the local
-	 * semantics exactly don't seem worthwhile (see also comments for
-	 * RowMarkType).
-	 *
-	 * Note: because we actually run the query as a cursor, this assumes that
-	 * DECLARE CURSOR ... FOR UPDATE is supported, which it isn't before 8.3.
-	 */
-	if (baserel->relid == root->parse->resultRelation &&
-		(root->parse->commandType == CMD_UPDATE ||
-		 root->parse->commandType == CMD_DELETE))
-	{
-		/* Relation is UPDATE/DELETE target, so use FOR UPDATE */
-		appendStringInfoString(&sql, " FOR UPDATE");
-	}
-	else
-	{
-		PlanRowMark *rc = get_plan_rowmark(root->rowMarks, baserel->relid);
-
-		if (rc)
-		{
-			/*
-			 * Relation is specified as a FOR UPDATE/SHARE target, so handle
-			 * that.  (But we could also see LCS_NONE, meaning this isn't a
-			 * target relation after all.)
-			 *
-			 * For now, just ignore any [NO] KEY specification, since (a) it's
-			 * not clear what that means for a remote table that we don't have
-			 * complete information about, and (b) it wouldn't work anyway on
-			 * older remote servers.  Likewise, we don't worry about NOWAIT.
-			 */
-			switch (rc->strength)
-			{
-				case LCS_NONE:
-					/* No locking needed */
-					break;
-				case LCS_FORKEYSHARE:
-				case LCS_FORSHARE:
-					appendStringInfoString(&sql, " FOR SHARE");
-					break;
-				case LCS_FORNOKEYUPDATE:
-				case LCS_FORUPDATE:
-					appendStringInfoString(&sql, " FOR UPDATE");
-					break;
-			}
-		}
-	}
+	/* Add any necessary FOR UPDATE/SHARE. */
+	deparseLockingClause(&sql, root, baserel);
 
 	/*
 	 * Build the fdw_private list that will be available to the executor.

contrib/postgres_fdw/postgres_fdw.h

@@ -88,6 +88,8 @@ extern void deparseSelectSql(StringInfo buf,
 				 RelOptInfo *baserel,
 				 Bitmapset *attrs_used,
 				 List **retrieved_attrs);
+extern void deparseLockingClause(StringInfo buf,
+					 PlannerInfo *root, RelOptInfo *rel);
 extern void appendWhereClause(StringInfo buf,
 				  PlannerInfo *root,
 				  RelOptInfo *baserel,