[parser] ast/functions: add tidb_decode_plan() function (#557)

Author: crazycs520

parser/ast/functions.go

@@ -246,6 +246,7 @@ const (
 	Version        = "version"
 	TiDBVersion    = "tidb_version"
 	TiDBIsDDLOwner = "tidb_is_ddl_owner"
+	TiDBDecodePlan = "tidb_decode_plan"
 
 	// control functions
 	If     = "if"

parser/parser_test.go

@@ -1198,6 +1198,7 @@ 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_plan();`, true, "SELECT TIDB_DECODE_PLAN()"},
 		{`SELECT tidb_decode_key('abc');`, true, "SELECT TIDB_DECODE_KEY('abc')"},
 
 		// for time fsp