vitessio/vitess · error

unable to select @@foreign_key_checks

Error message

unable to select @@foreign_key_checks

What it means

getSettingFKCheck reads the current @@foreign_key_checks value from the target mysqld before replicating (it saves the setting, disables FK checks during apply, and restores it). This error is thrown when the query succeeds but returns anything other than exactly one row and one column, which should never happen.

Source

Thrown at go/vt/vttablet/tabletmanager/vreplication/vreplicator.go:610

		return nil
	}
	insertLog(vr.dbClient, LogStateChange, vr.id, state.String(), message)
	vr.state = state

	return nil
}

func encodeString(in string) string {
	return sqltypes.EncodeStringSQL(in)
}

func (vr *vreplicator) getSettingFKCheck() error {
	qr, err := vr.dbClient.Execute("select @@foreign_key_checks")
	if err != nil {
		return err
	}
	if len(qr.Rows) != 1 || len(qr.Fields) != 1 {
		return errors.New("unable to select @@foreign_key_checks")
	}
	vr.originalFKCheckSetting, err = qr.Rows[0][0].ToCastInt64()
	if err != nil {
		return err
	}
	return nil
}

func (vr *vreplicator) needFKRestrict() bool {
	ok, err := vr.dbClient.SupportsCapability(capabilities.RestrictFKOnNonStandardKey)
	if err != nil {
		return false
	}
	return ok
}

func (vr *vreplicator) getSettingFKRestrict() error {
	if !vr.needFKRestrict() {

View on GitHub (pinned to 01a25a7d17)

Solutions

1. Check the target mysqld health and version — confirm `select @@foreign_key_checks` works in a mysql client on the same instance
2. Reset/reconnect the vreplication dbClient connection (restart the workflow or vttablet) if the connection returned a corrupted result
3. If behind a MySQL proxy (e.g. Vitess itself or another proxy), ensure it forwards system-variable SELECTs correctly

// Verify the target supports the variable before relying on FK-check save/restore
qr, err := conn.ExecuteFetch("SELECT @@foreign_key_checks", 1, false)
if err != nil || len(qr.Rows) != 1 || len(qr.Fields) != 1 {
    return fmt.Errorf("target cannot read @@foreign_key_checks")
}

if err := vr.getSettingFKCheck(); err != nil {
    if strings.Contains(err.Error(), "unable to select @@foreign_key_checks") {
        // reconnect dbClient and retry once
        vr.dbClient.Close()
        if rerr := vr.dbClient.Connect(); rerr == nil { return vr.getSettingFKCheck() }
    }
    return err
}

When it happens

Trigger: vr.dbClient.Execute("select @@foreign_key_checks") returns a result whose Rows/Fields cardinality is not 1x1 — e.g. an empty/odd result from a misbehaving connection or proxy.

Common situations: A connection pool/proxy mangles results; the dbClient connection is in a broken state mid-query; custom MySQL builds or versions where the system variable read behaves oddly.

Related errors

• unable to select @@session.restrict_fk_on_non_standard_key
• failed to set session foreign_key_checks: %w
• value out of range
• both atomic copy and partial mode cannot be specified for th
• invalid workflow

AI-assisted analysis of vitessio/vitess@01a25a7d17 (2026-09-01). Data as JSON: /api/errors/d1d72ad841b73e68. Report an issue: GitHub.