Attempt instant DDL if supported

Author: morgo

go/base/context.go

@@ -101,6 +101,7 @@ type MigrationContext struct {
 	AliyunRDS                bool
 	GoogleCloudPlatform      bool
 	AzureMySQL               bool
+	AttemptInstantDDL        bool
 
 	config            ContextConfig
 	configMutex       *sync.Mutex

go/cmd/gh-ost/main.go

@@ -67,6 +67,8 @@ func main() {
 	flag.StringVar(&migrationContext.DatabaseName, "database", "", "database name (mandatory)")
 	flag.StringVar(&migrationContext.OriginalTableName, "table", "", "table name (mandatory)")
 	flag.StringVar(&migrationContext.AlterStatement, "alter", "", "alter statement (mandatory)")
+	flag.BoolVar(&migrationContext.AttemptInstantDDL, "attempt-instant-ddl", true, "Attempt to use instant DDL for this migration first.")
+
 	flag.BoolVar(&migrationContext.CountTableRows, "exact-rowcount", false, "actually count table rows as opposed to estimate them (results in more accurate progress estimation)")
 	flag.BoolVar(&migrationContext.ConcurrentCountTableRows, "concurrent-rowcount", true, "(with --exact-rowcount), when true (default): count rows after row-copy begins, concurrently, and adjust row estimate later on; when false: first count rows, then start row copy")
 	flag.BoolVar(&migrationContext.AllowedRunningOnMaster, "allow-on-master", false, "allow this migration to run directly on master. Preferably it would run on a replica")

go/logic/applier.go

@@ -188,6 +188,54 @@ func (this *Applier) ValidateOrDropExistingTables() error {
 	return nil
 }
 
+// AttemptInstantDDL attempts to use instant DDL (from MySQL 8.0, and earlier in Aurora and some others.)
+// to apply the ALTER statement immediately. If it errors, the original
+// gh-ost algorithm can be used. However, if it's successful -- a lot
+// of time can potentially be saved. Instant operations include:
+// - Adding a column
+// - Dropping a column
+// - Dropping an index
+// - Extending a varchar column
+// It is safer to attempt the change than try and parse the DDL, since
+// there might be specifics about the table which make it not possible to apply instantly.
+func (this *Applier) AttemptInstantDDL() error {
+
+	query := fmt.Sprintf(`ALTER /* gh-ost */ TABLE %s.%s %s, ALGORITHM=INSTANT`,
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.OriginalTableName),
+		this.migrationContext.AlterStatementOptions,
+	)
+	this.migrationContext.Log.Infof("INSTANT DDL Query is: %s", query)
+
+	err := func() error {
+		tx, err := this.db.Begin()
+		if err != nil {
+			return err
+		}
+		defer tx.Rollback()
+
+		sessionQuery := fmt.Sprintf(`SET SESSION time_zone = '%s'`, this.migrationContext.ApplierTimeZone)
+		sessionQuery = fmt.Sprintf("%s, %s", sessionQuery, this.generateSqlModeQuery())
+
+		if _, err := tx.Exec(sessionQuery); err != nil {
+			return err
+		}
+		if _, err := tx.Exec(query); err != nil {
+			this.migrationContext.Log.Infof("INSTANT DDL failed: %s", err)
+			return err
+		}
+		if err := tx.Commit(); err != nil {
+			// Neither SET SESSION nor ALTER are really transactional, so strictly speaking
+			// there's no need to commit; but let's do this the legit way anyway.
+			return err
+		}
+		return nil
+	}()
+
+	return err
+
+}
+
 // CreateGhostTable creates the ghost table on the applier host
 func (this *Applier) CreateGhostTable() error {
 	query := fmt.Sprintf(`create /* gh-ost */ table %s.%s like %s.%s`,

go/logic/migrator.go

@@ -351,12 +351,26 @@ func (this *Migrator) Migrate() (err error) {
 	if err := this.initiateInspector(); err != nil {
 		return err
 	}
+	// In MySQL 8.0 (and possibly earlier) some DDL statements can be applied instantly.
+	// As just a metadata change. We can't detect this unless we attempt the statement
+	// (i.e. there is no explain for DDL).
+	if this.migrationContext.AttemptInstantDDL {
+		this.migrationContext.Log.Infof("Attempting to execute ALTER TABLE as INSTANT DDL")
+		if err := this.attemptInstantDDL(); err == nil {
+			this.migrationContext.Log.Infof("Success! Table %s.%s migrated instantly", sql.EscapeName(this.migrationContext.DatabaseName), sql.EscapeName(this.migrationContext.OriginalTableName))
+			return nil
+		} else {
+			this.migrationContext.Log.Infof("INSTANT DDL failed, will proceed with original algorithm: %s", err)
+		}
+	}
+
 	if err := this.initiateStreaming(); err != nil {
 		return err
 	}
 	if err := this.initiateApplier(); err != nil {
 		return err
 	}
+
 	if err := this.createFlagFiles(); err != nil {
 		return err
 	}
@@ -734,6 +748,17 @@ func (this *Migrator) initiateServer() (err error) {
 	return nil
 }
 
+// attemptInstantDDL tries to apply the DDL statement directly to the table
+// using a ALGORITHM=INSTANT assertion. If this fails, it will return an error,
+// in which case the original algorithm should be used.
+func (this *Migrator) attemptInstantDDL() (err error) {
+	this.applier = NewApplier(this.migrationContext)
+	if err := this.applier.InitDBConnections(); err != nil {
+		return err
+	}
+	return this.applier.AttemptInstantDDL()
+}
+
 // initiateInspector connects, validates and inspects the "inspector" server.
 // The "inspector" server is typically a replica; it is where we issue some
 // queries such as: