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.
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.
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).
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.
The basic format is this:
- The buffer is in-order of the index. First column in the index is first in the buffer, second second etc.
- The buffer must be zero-filled. The server kernel will use memcmp to compare two key values.
- If the column is NULLable, then the first byte is set to 1 if the column is null. Else, 0 means not-null.
- From ha_innodb.cc (for BLOBs, which I haven’t put in embedded_innodb yet): If the column is of a BLOB type (it must be a column prefix field in this case), then we put the length of the data in the field to the next 2 bytes, in the little-endian format. If the field is SQL NULL, then these 2 bytes are set to 0. Note that the length of data in the field is <= column prefix length.
- For fixed length fields (such as int), the next max field length bytes are for that field.
- For VARCHAR, there is always a 2 byte (in little endian) length. This is different to the row format, which may have 1 or 2 bytes. In the key tuple format it is ALWAYS two bytes.
I’ll discuss the use of this for rnd_pos() and position() in a later post…
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