MariaDB 5.3 query optimizer by Sergey Petrunia

Sergey Petrunia of the MariaDB project.

What exactly is not working in MySQL? MySQL is poor at decision support/analytics. With large datasets you need special disk access strategies. Complex queries like insufficient subquery support and big joins are common int he MySQL world.

DBT-3 is used, scale=30, with a 75GB database and run a query “average price of item between a range of dates”. Query time took some 45 minutes to execute. Why? Run iostat -x to see what is going on. See that the CPU is mostly idle, so its an IO-bound load. Next you run SHOW ENGINE INNODB STATUS and you’ll see how many reads per second is happening. Possible solution is to get more RAM or get an SSD (good to speedup OLTP workloads, but analytics over data is probably not viable since SSDs are small and not cheap).

The MySQL/MariaDB solution to the above problem is improved disk access strategies: multi-range read (MRR) and batched key access (BKA). In MariaDB, MRR/BKA need to be enabled (they are not turned on by default). The query time only took 3 minutes 48 seconds, which is some 11.8x faster than the previous 45 minutes.If you look at EXPLAIN output, its almost as same as before, expect the Extra filed. iostat -x will now show some CPU load, svctm down as well (so its not random disk seeks anymore — some 8ms seek time on a regular 7,200rpm disk), SHOW ENGINE INNODB STATUS will show some 10,000 reads per second rather than the previous 200.

If you are on Fedora, check out the Systemtap feature to look at I/O patterns. stap deviceseeks.stp -c “sleep 60″.

Subqueries handling in MariaDB 5.3: check out the Subquery Optimizations Map. Only about 10% of the audience use optimizer hints in MySQL.

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