feat: add an optimizer switch params to migration context

to be able to pass for example:

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

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