pg_dump: Lock all interesting tables in single statement.

This reduces the potentially significant command and
communication overhead between frontend and backend
when sending a LOCK TABLE query for each table.

Discussion: https://www.postgresql.org/message-id/20120530.180620.600165924826262795.t-ishii%40sraoss.co.jp

Authored-by: Brent Doil <bdoil@vmware.com>

Author: bmdoil

src/bin/pg_dump/pg_dump.c

@@ -7256,6 +7256,7 @@ getTables(Archive *fout, int *numTables)
 	int			i;
 	PQExpBuffer query = createPQExpBuffer();
 	TableInfo  *tblinfo;
+	bool 		lockTableDumped = false;
 	int			i_reltableoid;
 	int			i_reloid;
 	int			i_relname;
@@ -7963,6 +7964,8 @@ getTables(Archive *fout, int *numTables)
 		ExecuteSqlStatement(fout, query->data);
 	}
 
+	resetPQExpBuffer(query);
+
 	for (i = 0; i < ntups; i++)
 	{
 		tblinfo[i].dobj.objType = DO_TABLE;
@@ -8098,6 +8101,11 @@ getTables(Archive *fout, int *numTables)
 		 *
 		 * We only need to lock the table for certain components; see
 		 * pg_dump.h
+		 * 
+		 * GPDB: Build a single LOCK TABLE statement to lock all interesting tables.
+		 * This is more performant than issuing a separate LOCK TABLE statement for each table,
+		 * with considerable savings in FE/BE overhead. It does come at the cost of some increased
+		 * memory usage in both FE and BE, which we will be able to tolerate.
 		 */
 		/* GPDB_14_MERGE_FIXME: GPDB_96_MERGE_FIXME: Is the parrelid check still needed? */
 		if (tblinfo[i].dobj.dump &&
@@ -8106,18 +8114,29 @@ getTables(Archive *fout, int *numTables)
 			tblinfo[i].parrelid == 0 &&
 			(tblinfo[i].dobj.dump & DUMP_COMPONENTS_REQUIRING_LOCK))
 		{
-			resetPQExpBuffer(query);
-			appendPQExpBuffer(query,
-							  "LOCK TABLE %s IN ACCESS SHARE MODE",
-							  fmtQualifiedDumpable(&tblinfo[i]));
-			ExecuteSqlStatement(fout, query->data);
+			if (!lockTableDumped)
+				appendPQExpBuffer(query,
+						"LOCK TABLE %s ",
+						fmtQualifiedDumpable(&tblinfo[i]));
+			else
+				appendPQExpBuffer(query,
+						",%s ",
+						fmtQualifiedDumpable(&tblinfo[i]));
+			lockTableDumped = true;
 		}
 
 		/* Emit notice if join for owner failed */
 		if (strlen(tblinfo[i].rolname) == 0)
 			pg_log_warning("owner of table \"%s\" appears to be invalid",
 						   tblinfo[i].dobj.name);
 	}
+	/* Are there any tables to lock? */
+	if (lockTableDumped)
+	{
+	appendPQExpBuffer(query,
+			"IN ACCESS SHARE MODE");
+	ExecuteSqlStatement(fout, query->data);
+	}
 
 	if (dopt->lockWaitTimeout)
 	{