chore: ignore MySQL client sent SELECT $$ (#2896)

Signed-off-by: tison <wander4096@gmail.com>

src/servers/src/mysql/federated.rs

@@ -80,6 +80,9 @@ static OTHER_NOT_SUPPORTED_STMT: Lazy<RegexSet> = Lazy::new(|| {
         "(?i)^(SHOW COLLATION)",
         "(?i)^(SHOW CHARSET)",
 
+        // mysqlclient.
+        "(?i)^(SELECT \\$\\$)",
+
         // mysqldump.
         "(?i)^(SET SESSION(.*))",
         "(?i)^(SET SQL_QUOTE_SHOW_CREATE(.*))",