{"id":1876,"date":"2010-04-02T00:14:32","date_gmt":"2010-04-01T14:14:32","guid":{"rendered":"http:\/\/www.flamingspork.com\/blog\/?p=1876"},"modified":"2010-04-02T00:15:30","modified_gmt":"2010-04-01T14:15:30","slug":"the-drizzle-and-mysql-key-tuple-format","status":"publish","type":"post","link":"https:\/\/www.flamingspork.com\/blog\/2010\/04\/02\/the-drizzle-and-mysql-key-tuple-format\/","title":{"rendered":"The Drizzle (and MySQL) Key tuple format"},"content":{"rendered":"

Here’s something that’s not really documented anywhere (unless you count ha_innodb.cc as a source of server documentation). You may have some idea about the MySQL\/Drizzle row buffer format. This is passed around the storage engine interface: in for write_row and update_row and out for the various scan and index read methods.<\/p>\n

If you want to see the docs for it that exist in the code, check out store_key_val_for_row in ha_innodb.cc.<\/p>\n

However, there is another format that is passed to your engine (and that your engine is expected to understand) and for lack of a better name, I’m going to call it the key tuple format. The first place you’ll probably see this is when implementing the index_read function for a Cursor (or handler in MySQL speak).<\/p>\n

You get two things: a pointer to the buffer and the length of the buffer. Since a key can be made up of multiple parts, some of which can be NULL and some of which can be of variable length, this buffer is not (usually) a simple value. If you are starting out in your engine development, you can use this buffer blindly as a single value for non-nullable indexes with only 1 column.<\/p>\n

The basic format is this:<\/p>\n