diff --git a/ast/functions.go b/ast/functions.go
index 6a1f94016..74f09bf1b 100755
--- a/ast/functions.go
+++ b/ast/functions.go
@@ -321,6 +321,9 @@ const (
 	JSONDepth         = "json_depth"
 	JSONKeys          = "json_keys"
 	JSONLength        = "json_length"
+
+	// TiDB internal function.
+	TiDBDecodeKey = "tidb_decode_key"
 )
 
 // FuncCallExpr is for function expression.
diff --git a/parser_test.go b/parser_test.go
index 6c19ff853..6a47afc76 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -1174,6 +1174,7 @@ func (s *testParserSuite) TestBuiltin(c *C) {
 
 		{`SELECT tidb_version();`, true, "SELECT TIDB_VERSION()"},
 		{`SELECT tidb_is_ddl_owner();`, true, "SELECT TIDB_IS_DDL_OWNER()"},
+		{`SELECT tidb_decode_key('abc');`, true, "SELECT TIDB_DECODE_KEY('abc')"},
 
 		// for time fsp
 		{"CREATE TABLE t( c1 TIME(2), c2 DATETIME(2), c3 TIMESTAMP(2) );", true, "CREATE TABLE `t` (`c1` TIME(2),`c2` DATETIME(2),`c3` TIMESTAMP(2))"},