feat: add an optimizer switch params to migration context

to be able to pass for example:

`--optimizer-switch="prefer_ordering_index=on"`

Author: cyrinux

go/base/context.go

@@ -209,6 +209,7 @@ type MigrationContext struct {
 	CutOverCompleteFlag                    int64
 	InCutOverCriticalSectionFlag           int64
 	PanicAbort                             chan error
+	OptimizerSwitch                        string
 
 	OriginalTableColumnsOnApplier    *sql.ColumnList
 	OriginalTableColumns             *sql.ColumnList

go/cmd/gh-ost/main.go

@@ -139,6 +139,7 @@ func main() {
 	criticalLoad := flag.String("critical-load", "", "Comma delimited status-name=threshold, same format as --max-load. When status exceeds threshold, app panics and quits")
 	flag.Int64Var(&migrationContext.CriticalLoadIntervalMilliseconds, "critical-load-interval-millis", 0, "When 0, migration immediately bails out upon meeting critical-load. When non-zero, a second check is done after given interval, and migration only bails out if 2nd check still meets critical load")
 	flag.Int64Var(&migrationContext.CriticalLoadHibernateSeconds, "critical-load-hibernate-seconds", 0, "When non-zero, critical-load does not panic and bail out; instead, gh-ost goes into hibernation for the specified duration. It will not read/write anything from/to any server")
+	flag.StringVar(&migrationContext.OptimizerSwitch, "optimizer-switch", "", "Optimizer switch params, eg: prefer_ordering_index=on")
 	quiet := flag.Bool("quiet", false, "quiet")
 	verbose := flag.Bool("verbose", false, "verbose")
 	debug := flag.Bool("debug", false, "debug mode (very verbose)")

go/logic/applier.go

@@ -49,6 +49,15 @@ func newDmlBuildResultError(err error) *dmlBuildResult {
 	}
 }
 
+func (this *Applier) setOptimizer(tx *gosql.Tx) error {
+	if this.migrationContext.OptimizerSwitch == "" {
+		return nil
+	}
+	optimizerString := fmt.Sprintf("set session optimizer_switch=%q", this.migrationContext.OptimizerSwitch)
+	_, err := tx.Query(optimizerString)
+	return err
+}
+
 // Applier connects and writes the applier-server, which is the server where migration
 // happens. This is typically the master, but could be a replica when `--test-on-replica` or
 // `--execute-on-replica` are given.
@@ -477,6 +486,11 @@ func (this *Applier) readMigrationMinValues(tx *gosql.Tx, uniqueKey *sql.UniqueK
 		return err
 	}
 
+	err = this.setOptimizer(tx)
+	if err != nil {
+		return err
+	}
+
 	rows, err := tx.Query(query)
 	if err != nil {
 		return err
@@ -502,6 +516,11 @@ func (this *Applier) readMigrationMaxValues(tx *gosql.Tx, uniqueKey *sql.UniqueK
 		return err
 	}
 
+	err = this.setOptimizer(tx)
+	if err != nil {
+		return err
+	}
+
 	rows, err := tx.Query(query)
 	if err != nil {
 		return err