Avoid multiple foreign server connections when all use same user mapping.

Previously, postgres_fdw's connection cache was keyed by user OID and
server OID, but this can lead to multiple connections when it's not
really necessary.  In particular, if all relevant users are mapped to
the public user mapping, then their connection options are certainly
the same, so one connection can be used for all of them.

While we're cleaning things up here, drop the "server" argument to
GetConnection(), which isn't really needed.  This saves a few cycles
because callers no longer have to look this up; the function itself
does, but only when establishing a new connection, not when reusing
an existing one.

Ashutosh Bapat, with a few small changes by me.

contrib/postgres_fdw/connection.c

@@ -24,9 +24,11 @@
 /*
  * Connection cache hash table entry
  *
- * The lookup key in this hash table is the foreign server OID plus the user
- * mapping OID.  (We use just one connection per user per foreign server,
- * so that we can ensure all scans use the same snapshot during a query.)
+ * The lookup key in this hash table is the user mapping OID. We use just one
+ * connection per user mapping ID, which ensures that all the scans use the
+ * same snapshot during a query.  Using the user mapping OID rather than
+ * the foreign server OID + user OID avoids creating multiple connections when
+ * the public user mapping applies to all user OIDs.
  *
  * The "conn" pointer can be NULL if we don't currently have a live connection.
  * When we do have a connection, xact_depth tracks the current depth of
@@ -35,11 +37,7 @@
  * ourselves, so that rolling back a subtransaction will kill the right
  * queries and not the wrong ones.
  */
-typedef struct ConnCacheKey
-{
-	Oid			serverid;		/* OID of foreign server */
-	Oid			userid;			/* OID of local user whose mapping we use */
-} ConnCacheKey;
+typedef Oid ConnCacheKey;
 
 typedef struct ConnCacheEntry
 {
@@ -94,8 +92,7 @@ static void pgfdw_subxact_callback(SubXactEvent event,
  * mid-transaction anyway.
  */
 PGconn *
-GetConnection(ForeignServer *server, UserMapping *user,
-			  bool will_prep_stmt)
+GetConnection(UserMapping *user, bool will_prep_stmt)
 {
 	bool		found;
 	ConnCacheEntry *entry;
@@ -127,8 +124,7 @@ GetConnection(ForeignServer *server, UserMapping *user,
 	xact_got_connection = true;
 
 	/* Create hash key for the entry.  Assume no pad bytes in key struct */
-	key.serverid = server->serverid;
-	key.userid = user->userid;
+	key = user->umid;
 
 	/*
 	 * Find or create cached entry for requested connection.
@@ -156,12 +152,15 @@ GetConnection(ForeignServer *server, UserMapping *user,
 	 */
 	if (entry->conn == NULL)
 	{
+		ForeignServer *server = GetForeignServer(user->serverid);
+
 		entry->xact_depth = 0;	/* just to be sure */
 		entry->have_prep_stmt = false;
 		entry->have_error = false;
 		entry->conn = connect_pg_server(server, user);
-		elog(DEBUG3, "new postgres_fdw connection %p for server \"%s\"",
-			 entry->conn, server->servername);
+
+		elog(DEBUG3, "new postgres_fdw connection %p for server \"%s\ (user mapping oid %d, userid %d)",
+			 entry->conn, server->servername, user->umid, user->userid);
 	}
 
 	/*

contrib/postgres_fdw/postgres_fdw.c

@@ -1101,7 +1101,6 @@ postgresBeginForeignScan(ForeignScanState *node, int eflags)
 	RangeTblEntry *rte;
 	Oid			userid;
 	ForeignTable *table;
-	ForeignServer *server;
 	UserMapping *user;
 	int			numParams;
 	int			i;
@@ -1129,14 +1128,13 @@ postgresBeginForeignScan(ForeignScanState *node, int eflags)
 	/* Get info about foreign table. */
 	fsstate->rel = node->ss.ss_currentRelation;
 	table = GetForeignTable(RelationGetRelid(fsstate->rel));
-	server = GetForeignServer(table->serverid);
-	user = GetUserMapping(userid, server->serverid);
+	user = GetUserMapping(userid, table->serverid);
 
 	/*
 	 * Get connection to the foreign server.  Connection manager will
 	 * establish new connection if necessary.
 	 */
-	fsstate->conn = GetConnection(server, user, false);
+	fsstate->conn = GetConnection(user, false);
 
 	/* Assign a unique ID for my cursor */
 	fsstate->cursor_number = GetCursorNumber(fsstate->conn);
@@ -1503,7 +1501,6 @@ postgresBeginForeignModify(ModifyTableState *mtstate,
 	RangeTblEntry *rte;
 	Oid			userid;
 	ForeignTable *table;
-	ForeignServer *server;
 	UserMapping *user;
 	AttrNumber	n_params;
 	Oid			typefnoid;
@@ -1530,11 +1527,10 @@ postgresBeginForeignModify(ModifyTableState *mtstate,
 
 	/* Get info about foreign table. */
 	table = GetForeignTable(RelationGetRelid(rel));
-	server = GetForeignServer(table->serverid);
-	user = GetUserMapping(userid, server->serverid);
+	user = GetUserMapping(userid, table->serverid);
 
 	/* Open connection; report that we'll create a prepared statement. */
-	fmstate->conn = GetConnection(server, user, true);
+	fmstate->conn = GetConnection(user, true);
 	fmstate->p_name = NULL;		/* prepared statement not made yet */
 
 	/* Deconstruct fdw_private data. */
@@ -1988,7 +1984,7 @@ estimate_path_cost_size(PlannerInfo *root,
 			appendOrderByClause(&sql, root, baserel, pathkeys);
 
 		/* Get the remote estimate */
-		conn = GetConnection(fpinfo->server, fpinfo->user, false);
+		conn = GetConnection(fpinfo->user, false);
 		get_remote_estimate(sql.data, conn, &rows, &width,
 							&startup_cost, &total_cost);
 		ReleaseConnection(conn);
@@ -2544,7 +2540,6 @@ postgresAnalyzeForeignTable(Relation relation,
 							BlockNumber *totalpages)
 {
 	ForeignTable *table;
-	ForeignServer *server;
 	UserMapping *user;
 	PGconn	   *conn;
 	StringInfoData sql;
@@ -2565,9 +2560,8 @@ postgresAnalyzeForeignTable(Relation relation,
 	 * owner, even if the ANALYZE was started by some other user.
 	 */
 	table = GetForeignTable(RelationGetRelid(relation));
-	server = GetForeignServer(table->serverid);
-	user = GetUserMapping(relation->rd_rel->relowner, server->serverid);
-	conn = GetConnection(server, user, false);
+	user = GetUserMapping(relation->rd_rel->relowner, table->serverid);
+	conn = GetConnection(user, false);
 
 	/*
 	 * Construct command to get page count for relation.
@@ -2626,7 +2620,6 @@ postgresAcquireSampleRowsFunc(Relation relation, int elevel,
 {
 	PgFdwAnalyzeState astate;
 	ForeignTable *table;
-	ForeignServer *server;
 	UserMapping *user;
 	PGconn	   *conn;
 	unsigned int cursor_number;
@@ -2657,9 +2650,8 @@ postgresAcquireSampleRowsFunc(Relation relation, int elevel,
 	 * owner, even if the ANALYZE was started by some other user.
 	 */
 	table = GetForeignTable(RelationGetRelid(relation));
-	server = GetForeignServer(table->serverid);
-	user = GetUserMapping(relation->rd_rel->relowner, server->serverid);
-	conn = GetConnection(server, user, false);
+	user = GetUserMapping(relation->rd_rel->relowner, table->serverid);
+	conn = GetConnection(user, false);
 
 	/*
 	 * Construct cursor that retrieves whole rows from remote.
@@ -2860,7 +2852,7 @@ postgresImportForeignSchema(ImportForeignSchemaStmt *stmt, Oid serverOid)
 	 */
 	server = GetForeignServer(serverOid);
 	mapping = GetUserMapping(GetUserId(), server->serverid);
-	conn = GetConnection(server, mapping, false);
+	conn = GetConnection(mapping, false);
 
 	/* Don't attempt to import collation if remote server hasn't got it */
 	if (PQserverVersion(conn) < 90100)

contrib/postgres_fdw/postgres_fdw.h

@@ -60,8 +60,7 @@ extern int	set_transmission_modes(void);
 extern void reset_transmission_modes(int nestlevel);
 
 /* in connection.c */
-extern PGconn *GetConnection(ForeignServer *server, UserMapping *user,
-			  bool will_prep_stmt);
+extern PGconn *GetConnection(UserMapping *user, bool will_prep_stmt);
 extern void ReleaseConnection(PGconn *conn);
 extern unsigned int GetCursorNumber(PGconn *conn);
 extern unsigned int GetPrepStmtNumber(PGconn *conn);

src/backend/foreign/foreign.c

@@ -193,6 +193,7 @@ GetUserMapping(Oid userid, Oid serverid)
 						MappingUserName(userid))));
 
 	um = (UserMapping *) palloc(sizeof(UserMapping));
+	um->umid = HeapTupleGetOid(tp);
 	um->userid = userid;
 	um->serverid = serverid;
 

src/include/foreign/foreign.h

@@ -55,6 +55,7 @@ typedef struct ForeignServer
 
 typedef struct UserMapping
 {
+	Oid			umid;			/* Oid of user mapping */
 	Oid			userid;			/* local user Oid */
 	Oid			serverid;		/* server Oid */
 	List	   *options;		/* useoptions as DefElem list */