Yet another Database Blog

Using EXPLAIN EXTENDED to see view query rewrites

At the MySQL Mini Conference in Sydney this week we discussed how to use EXPLAIN EXTENDED to view the rewrites undertaken by the MySQL optimizer.  IN particular, to see if MySQL performs a merge of the query into the view definition, or if it creates a temporary table.

D.I.Y. MySQL 5.1 monitoring

I wrote recently about using events and the new processlist table in MySQL 5.1 to keep track of the number of connected processes.  Although having the PROCESSLIST available as an INFORMATION SCHEMA table is usefull, it seemed to me that having SHOW GLOBAL STATUS exposed in a similar fashion would be far more useful.  So at the MySQL UC last year, I asked Brian Aker if that would be possible.  I know how many enhancement requests MySQL has to deal with, so I was really happy to see that table appear in the latest 5.1 build (5.1.14 beta). 

This table, together with the EVENT scheduler, lets us keep track of the values of status variables over time without having to have any external deamon running.  This won't come anywhere near to matching what MySQL have made avalable in Merlin, but still could be fairly useful.  So lets build a simple system using events to keep track of "interesting" status variables....

Unit testing stored procedures with Junit

Anyone who has used an automated unit testing framework such as Junit knows just how life-changing an automated test suite can be.   Once you've  experienced validating that recent changes have not broken old code, or discovering subtle bugs via junit that would otherwise have remained undetected , you naturally want to have this capability in all your programming environments.

Guisseppe Maxia has written a few stored procedure snippets to assist with automated unit testing of MySQL routines.  Unfortunately, the MySQL stored procedure language itself does not have the necessary abilities to fully implement the sort of unit testing we would like.  In particular, the inability for a stored procedure to capture the result sets generated by another stored procedure prevents a stored procedure from fully unit testing another. 

So I decided that - for me - Junit offered the best solution.  I created an extension to the Junit class that contains some assertions useful when unit testing stored procedure and extended my PlainSql JDBC wrapper classes to allow a stored procedure to return a single object that contains all its result sets and the values of OUT or INOUT parameters.   This object can be made the target of the various assertions.

If you're not familiar with Java and/or, you might feel that this solution is not for you.  However, the amount of java programming you need to do is very minimal and GUI environments such as Eclipse make it very easy to set up.  The rest of this article is available here;  it contains instructions, my Java classes and examples for setting up Junit test cases for MySQL stored procedures.

MySQL 5.1 events

I finally got around to working with the 5.1 scheduler.  I wanted to have a simple but non-trivial example, and when I saw Brian Akers post on the new processlist table, I thought of a useful little application:  I would submit an event that would summarize the users and their statuses at regular intervals so I could track user trends.

First off,  I needed to enable the scheduler by adding the following line to my configuration file:

event_scheduler=1

Now the scheduler is ready for action.  So I created a table to hold my process list details:

CREATE TABLE processhistory (h_timestamp DATETIME,
                             processcount INTEGER,
                             activecount INTEGER,
                             lockedcount INTEGER)$$

And then the DML to create an event:

CREATE EVENT evt_process_history
     ON SCHEDULE EVERY 2 MINUTE
     DO
BEGIN
    INSERT INTO processhistory (h_timestamp,processcount,
            activecount,lockedcount)
     SELECT NOW() AS h_timestamp,COUNT(*) AS processcount,
            SUM(active) AS activecount  ,
            SUM(locked) AS lockedcount
       FROM (SELECT CASE command WHEN 'Sleep' THEN 0 ELSE 1
                     END AS active ,
                    CASE state WHEN 'Locked' THEN 1 ELSE 0
                     END AS locked
               FROM information_schema.`PROCESSLIST` P) Q;
END$$

Every two minutes, the event summarizes the status of the sessions currently connected and stores them to the table.  I could use various tools to analze this data, but for convenience I used Excel with the ODBC driver to create a chart of activity:

Cool! Now I can keep track of active sessions over time, which could be useful. On the test database, there is a little ruby program that locks up a table needed by my java TP simulation, so we see those spikes of lock activity. I'm hoping that MySQL expose the SHOW STATUS command as a table as well, since we can't get at the contents of SHOW STATUS from within the stored program language.