Add XML functions to incompatible (#668)

morgo · ngaut · commit d97cea7ac33a · 2018-10-16T10:43:59.000+08:00
diff --git a/sql/mysql-compatibility.md b/sql/mysql-compatibility.md
@@ -27,6 +27,7 @@ However, in TiDB, the following MySQL features are not supported for the time be
 + Drop primary key
 + SYS schema
 + Optimizer trace
++ XML Functions
 + X-Protocol
 
 ## Features that are different from MySQL
