add rewriteBatchedStatements=true to MySQL jdbc driver config

frameworks/Groovy/grails/hello/grails-app/conf/mysql-connection.properties

@@ -58,3 +58,6 @@ maintainTimeStats=false
 # timeouts for TCP/IP
 connectTimeout=15000
 socketTimeout=120000
+
+# enable batch mode
+rewriteBatchedStatements=true