OnlineDDL: skip GetSchema where possible (vitessio#10107)

Signed-off-by: Shlomi Noach <[email protected]>

go/vt/schemamanager/tablet_executor.go

@@ -217,6 +217,17 @@ func (exec *TabletExecutor) isOnlineSchemaDDL(stmt sqlparser.Statement) (isOnlin
 //   1. Alter more than 100,000 rows.
 //   2. Change a table with more than 2,000,000 rows (Drops are fine).
 func (exec *TabletExecutor) detectBigSchemaChanges(ctx context.Context, parsedDDLs []sqlparser.DDLStatement) (bool, error) {
+	// We want to avoid any overhead if possible. If all DDLs are online schema changes, then we want to
+	// skip GetSchema altogether.
+	foundAnyNonOnlineDDL := false
+	for _, ddl := range parsedDDLs {
+		if !exec.isOnlineSchemaDDL(ddl) {
+			foundAnyNonOnlineDDL = true
+		}
+	}
+	if !foundAnyNonOnlineDDL {
+		return false, nil
+	}
 	// exec.tablets is guaranteed to have at least one element;
 	// Otherwise, Open should fail and executor should fail.
 	primaryTabletInfo := exec.tablets[0]