Toru Maesaka: Drizzle Storage Engine Dev: Determining Query Type
Determining what kind of SQL query is requested at the handler level is pretty important for BlitzDB since the strategy is to obtain the most suitable lock for a given request. Unfortunately there is no intuitive way to get this information. So, I took a peek into InnoDB’s sourcecode and found my solution (open source saves the day as usual).
Solution
In Drizzle, there is a function called session_sql_command(Session *session) which returns an integer that corresponds to one of the command type constants (which are accessible from the engine). Ideally I would like to call this function from anywhere in the engine but since it requires a session object as an argument, I could only call it from store_lock().
My solution was to add a variable in the handler class and assign the appropriate value to it from store_lock(). This turned out to be okay since store_lock() is called before any other API functions but the concern here is that store_lock() is planned for removal in the future.
Now I can do things like:
ha_blitz::rnd_init(bool drizzled_will_scan) { if (sql_command_type == SQLCOM_UPDATE) /* get the most suitable lock type for this task */ else if (sql_command_type == SQLCOM_SELECT) /* get the most suitable lock type for this task */ ... }
Personal Request to Drizzle
Although I would like to see store_lock() disappear from the storage engine API, I would like storage engines (technically worker threads) to have an ability to gather meta information on the query before any real work is done.
My request is for store_lock() to become something along the line of gather_information() where it gives the handler (or worker threads) a chance to gather information about the query. Needless to say, drizzled must call this function before any other API calls are made.
URL: http://torum.net/2009/11/drizzle-engine-query-type/
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