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Re: Stewart Smith: The Drizzle (and MySQL) Key tuple format

 

Hi,

On Fri, Apr 2, 2010 at 2:57 PM, 叶蔚 <cuteyw@xxxxxxxxx> wrote:

>  hi stewart, I read your post about key tuple format. But I still do
> not quite understand when and how will those index_foo functions will
> be called? I've looked through csv and archive engine, as well as
> mysql filesystem and awss3 engine. None of then support index. I try
> to figure it out by reading source code of innodb engine, but still
> dose not quite understand. So I have no idea about how to implement
> index in storage engine. Can you give me some hints?
>

In my opinion, I don't think we should support index on this file system
storage engine.
This file system engine should be simple and easy to use.
However, for the cloud service engine, we may need to support the index in
order to suppress the possible bandwidth.


>  And I did not read transaction-related source code  in depth. I am
> not sure whether I can implement a filesystem or could-based
> TransactionalStorageEngine in three-months GSOC. What's your opinion?
>
>  Thanks!
>
> 2010/4/2 Planet Drizzle <emailer@xxxxxxxxxxxxxxxxx>
> >
> > Stewart Smith: The Drizzle (and MySQL) Key tuple format
> >
> > 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…
> >
> > This blog post (but not the whole blog) is published under the Creative
> Commons Attribution-Share Alike License. Attribution is by linking back to
> this post and mentioning my name (Stewart Smith).
> >
> > URL:
> http://www.flamingspork.com/blog/2010/04/02/the-drizzle-and-mysql-key-tuple-format/
> >
> >
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