Hi, Sergey! On May 09, Sergey Vojtovich wrote:
@@ -4240,9 +4233,17 @@ thr_lock_type read_lock_type_for_table(THD *thd, DBUG_RETURN(true); }
- if (! (flags & MYSQL_OPEN_SKIP_SCOPED_MDL_LOCK) && - schema_set.insert(table)) - DBUG_RETURN(TRUE); + /* Scoped locks: Take intention exclusive locks on all involved schemas. */ + if (!(flags & MYSQL_OPEN_SKIP_SCOPED_MDL_LOCK)) + { + MDL_request *schema_request= new (thd->mem_root) MDL_request; + if (schema_request == NULL) + DBUG_RETURN(TRUE); + schema_request->init(MDL_key::SCHEMA, table->db, "", + MDL_INTENTION_EXCLUSIVE, + MDL_TRANSACTION); + mdl_requests.push_front(schema_request); + }
As far as I understand, the Hash_set was used here to make only one MDL request per schema. Now if you'll have 10 tables all in one schema, you'll make push 10 MDL requests instead of one. It was probably supposed to, but since Hash_set is non-unique it didn't do anything useful. That is we would get 10 MDL request with old code too.
Oh, ok.
Which may be fine, if MDL subsystem places only one lock in this case (iirc, it does). But what is cheaper - collapse identical schemas into one mdl request here (with Hash_set) or let mdl sort it out later? MDL doesn't eliminate them, but rather clones compatible requests. Performance wise it will be slower in certain cases, but since this is DDL is performance that important?
If it isn't, why are you eliminating Hash_set? Regards, Sergei