vacuumlo: Use a cursor to limit client-side memory usage.

This prevents the client from gobbling up too much memory when the
number of large objects to be removed is very large.

Andrew Dunstan, reviewed by Josh Kupershmidt

contrib/vacuumlo/vacuumlo.c

@@ -290,74 +290,101 @@ vacuumlo(const char *database, const struct _param * param)
 	PQclear(res);
 
 	buf[0] = '\0';
-	strcat(buf, "SELECT lo FROM vacuum_l");
+	strcat(buf,
+		   "DECLARE myportal CURSOR WITH HOLD FOR SELECT lo FROM vacuum_l");
 	res = PQexec(conn, buf);
-	if (PQresultStatus(res) != PGRES_TUPLES_OK)
+	if (PQresultStatus(res) != PGRES_COMMAND_OK)
 	{
-		fprintf(stderr, "Failed to read temp table:\n");
-		fprintf(stderr, "%s", PQerrorMessage(conn));
+		fprintf(stderr, "DECLARE CURSOR failed: %s", PQerrorMessage(conn));
 		PQclear(res);
 		PQfinish(conn);
 		return -1;
 	}
+	PQclear(res);
+
+	snprintf(buf, BUFSIZE, "FETCH FORWARD %ld IN myportal",
+			 param->transaction_limit > 0 ? param->transaction_limit : 1000L);
 
-	matched = PQntuples(res);
 	deleted = 0;
-	for (i = 0; i < matched; i++)
+
+	while (1)
 	{
-		Oid			lo = atooid(PQgetvalue(res, i, 0));
+		res = PQexec(conn, buf);
+		if (PQresultStatus(res) != PGRES_TUPLES_OK)
+		{
+			fprintf(stderr, "FETCH FORWARD failed: %s", PQerrorMessage(conn));
+			PQclear(res);
+			PQfinish(conn);
+			return -1;
+		}
 
-		if (param->verbose)
+		matched = PQntuples(res);
+		if (matched <= 0)
 		{
-			fprintf(stdout, "\rRemoving lo %6u   ", lo);
-			fflush(stdout);
+			/* at end of resultset */
+			PQclear(res);
+			break;
 		}
 
-		if (param->dry_run == 0)
+		for (i = 0; i < matched; i++)
 		{
-			if (lo_unlink(conn, lo) < 0)
+			Oid			lo = atooid(PQgetvalue(res, i, 0));
+
+			if (param->verbose)
+			{
+				fprintf(stdout, "\rRemoving lo %6u   ", lo);
+				fflush(stdout);
+			}
+
+			if (param->dry_run == 0)
 			{
-				fprintf(stderr, "\nFailed to remove lo %u: ", lo);
-				fprintf(stderr, "%s", PQerrorMessage(conn));
-				if (PQtransactionStatus(conn) == PQTRANS_INERROR)
+				if (lo_unlink(conn, lo) < 0)
 				{
-					success = false;
-					break;
+					fprintf(stderr, "\nFailed to remove lo %u: ", lo);
+					fprintf(stderr, "%s", PQerrorMessage(conn));
+					if (PQtransactionStatus(conn) == PQTRANS_INERROR)
+					{
+						success = false;
+						PQclear(res);
+						break;
+					}
 				}
+				else
+					deleted++;
 			}
 			else
 				deleted++;
-		}
-		else
-			deleted++;
-		if (param->transaction_limit > 0 &&
-			(deleted % param->transaction_limit) == 0)
-		{
-			res2 = PQexec(conn, "commit");
-			if (PQresultStatus(res2) != PGRES_COMMAND_OK)
+
+			if (param->transaction_limit > 0 &&
+				(deleted % param->transaction_limit) == 0)
 			{
-				fprintf(stderr, "Failed to commit transaction:\n");
-				fprintf(stderr, "%s", PQerrorMessage(conn));
+				res2 = PQexec(conn, "commit");
+				if (PQresultStatus(res2) != PGRES_COMMAND_OK)
+				{
+					fprintf(stderr, "Failed to commit transaction:\n");
+					fprintf(stderr, "%s", PQerrorMessage(conn));
+					PQclear(res2);
+					PQclear(res);
+					PQfinish(conn);
+					return -1;
+				}
 				PQclear(res2);
-				PQclear(res);
-				PQfinish(conn);
-				return -1;
-			}
-			PQclear(res2);
-			res2 = PQexec(conn, "begin");
-			if (PQresultStatus(res2) != PGRES_COMMAND_OK)
-			{
-				fprintf(stderr, "Failed to start transaction:\n");
-				fprintf(stderr, "%s", PQerrorMessage(conn));
+				res2 = PQexec(conn, "begin");
+				if (PQresultStatus(res2) != PGRES_COMMAND_OK)
+				{
+					fprintf(stderr, "Failed to start transaction:\n");
+					fprintf(stderr, "%s", PQerrorMessage(conn));
+					PQclear(res2);
+					PQclear(res);
+					PQfinish(conn);
+					return -1;
+				}
 				PQclear(res2);
-				PQclear(res);
-				PQfinish(conn);
-				return -1;
 			}
-			PQclear(res2);
 		}
+
+		PQclear(res);
 	}
-	PQclear(res);
 
 	/*
 	 * That's all folks!