support for --skip-foreign-key-checks

doc/command-line-flags.md

@@ -63,6 +63,8 @@ Optional. Default is `safe`. See more discussion in [cut-over](cut-over.md)
 
 At this time (10-2016) `gh-ost` does not support foreign keys on migrated tables (it bails out when it notices a FK on the migrated table). However, it is able to support _dropping_ of foreign keys via this flag. If you're trying to get rid of foreign keys in your environment, this is a useful flag.
 
+See also: [`skip-foreign-key-checks`](#skip-foreign-key-checks)
+
 ### exact-rowcount
 
 A `gh-ost` execution need to copy whatever rows you have in your existing table onto the ghost table. This can, and often be, a large number. Exactly what that number is?
@@ -109,6 +111,10 @@ See also: [Sub-second replication lag throttling](subsecond-lag.md)
 
 Typically `gh-ost` is used to migrate tables on a master. If you wish to only perform the migration in full on a replica, connect `gh-ost` to said replica and pass `--migrate-on-replica`. `gh-ost` will briefly connect to the master but other issue no changes on the master. Migration will be fully executed on the replica, while making sure to maintain a small replication lag.
 
+### skip-foreign-key-checks
+
+By default `gh-ost` verifies no foreign keys exist on the migrated table. On servers with large number of tables this check can take a long time. If you're absolutely certain no foreign keys exist (table does not referenece other table nor is referenced by other tables) and wish to save the check time, provide with `--skip-foreign-key-checks`.
+
 ### skip-renamed-columns
 
 See `approve-renamed-columns`

go/base/context.go

@@ -75,6 +75,7 @@ type MigrationContext struct {
 	AllowedMasterMaster      bool
 	SwitchToRowBinlogFormat  bool
 	AssumeRBR                bool
+	SkipForeignKeyChecks     bool
 	NullableUniqueKeyAllowed bool
 	ApproveRenamedColumns    bool
 	SkipRenamedColumns       bool

go/cmd/gh-ost/main.go

@@ -62,6 +62,7 @@ func main() {
 	flag.BoolVar(&migrationContext.SkipRenamedColumns, "skip-renamed-columns", false, "in case your `ALTER` statement renames columns, gh-ost will note that and offer its interpretation of the rename. By default gh-ost does not proceed to execute. This flag tells gh-ost to skip the renamed columns, i.e. to treat what gh-ost thinks are renamed columns as unrelated columns. NOTE: you may lose column data")
 	flag.BoolVar(&migrationContext.IsTungsten, "tungsten", false, "explicitly let gh-ost know that you are running on a tungsten-replication based topology (you are likely to also provide --assume-master-host)")
 	flag.BoolVar(&migrationContext.DiscardForeignKeys, "discard-foreign-keys", false, "DANGER! This flag will migrate a table that has foreign keys and will NOT create foreign keys on the ghost table, thus your altered table will have NO foreign keys. This is useful for intentional dropping of foreign keys")
+	flag.BoolVar(&migrationContext.SkipForeignKeyChecks, "skip-foreign-key-checks", false, "set to 'true' when you know for certain there are no foreign keys on your table, and wish to skip the time it takes for gh-ost to verify that")
 
 	executeFlag := flag.Bool("execute", false, "actually execute the alter & migrate the table. Default is noop: do some tests and exit")
 	flag.BoolVar(&migrationContext.TestOnReplica, "test-on-replica", false, "Have the migration run on a replica, not on the master. At the end of migration replication is stopped, and tables are swapped and immediately swap-revert. Replication remains stopped and you can compare the two tables for building trust")

go/logic/inspect.go

@@ -388,6 +388,10 @@ func (this *Inspector) validateTable() error {
 
 // validateTableForeignKeys makes sure no foreign keys exist on the migrated table
 func (this *Inspector) validateTableForeignKeys(allowChildForeignKeys bool) error {
+	if this.migrationContext.SkipForeignKeyChecks {
+		log.Warning("--skip-foreign-key-checks provided: will not check for foreign keys")
+		return nil
+	}
 	query := `
 		SELECT
 			SUM(REFERENCED_TABLE_NAME IS NOT NULL AND TABLE_SCHEMA=? AND TABLE_NAME=?) as num_child_side_fk,