Online DDL: ensure requested_timestamp isn't zero, before initializing table schema (#12263)

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

go/vt/vttablet/onlineddl/schema.go

@@ -393,6 +393,12 @@ const (
 			AND cleanup_timestamp IS NULL
 			AND completed_timestamp IS NULL
 	`
+	sqlFixRequestedTimestamp = `UPDATE _vt.schema_migrations
+		SET
+			requested_timestamp = added_timestamp
+		WHERE
+			requested_timestamp < added_timestamp;
+	`
 	sqlSelectMigration = `SELECT
 			id,
 			migration_uuid,
@@ -612,6 +618,13 @@ var (
 var ApplyDDL = []string{
 	sqlCreateSidecarDB,
 	sqlCreateSchemaMigrationsTable,
+	// Fixing a historical issue: past values of requested_timestamp could be '0000-00-00 00:00:00'.
+	// In turn, those cause `ERROR 1292 (22007): Incorrect datetime value` when attempting to
+	// make any DDL on the table.
+	// We trust added_timestamp to be non-zero (it defaults CURRENT_TIMESTAMP and never modified),
+	// and so we set requested_timestamp to that value.
+	// The query makes a full table scan, because neither column is indexed.
+	sqlFixRequestedTimestamp, // end  of fix
 	alterSchemaMigrationsTableRetries,
 	alterSchemaMigrationsTableTablet,
 	alterSchemaMigrationsTableArtifacts,