From: Kristian Nielsen
Date: May 6 2009 7:00am
Subject: Re: Questions about write_row and rnd_next
List-Archive: http://lists.mysql.com/internals/36589
Message-Id: <87hbzyk95k.fsf@knielsen-hq.org>
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Vyacheslav Akhmechet <coffeemug@stripped> writes:

> On Wed, May 6, 2009 at 1:38 AM, Kristian Nielsen
> <knielsen@stripped> wrote:
>> Hope this helps
> Thank you, this really helps. Am I correct to assume that table->field
> is simply a pointer to structures that are stored in the buffer passed
> to write_row (a cast, essentially)? When I access the fields from

No, this does not sound right.

> write_row I get the correct values, but I am not sure what the
> relationship between these data structures and the buffer passed to
> write_row is.

This is really central to how the storage handler interface works. You need to
study the code yourself a bit to work efficiently with it I think. From
memory, the basic idea is this:

The buffer passed to write_row etc. is a fixed-size buffer of bytes holding
the data for one row. It has a certain format, basically it contains NULL bits
and fractional bitfields, followed by data for each column in sequence, padded
to maximum length so the offsets of a field are constant. This is often called
a "record".

The table->field list has an entry for each column in the table. Each entry is
a class with methods for accessing the data of that field stored inside a
record. This list is constant for the table, it is kind of the metadata for
the columns.

The table has two pre-allocated buffers for records, table->record[0] and
table->record[1]. They are used for most record data to avoid malloc()
overhead. So usually the buffer passed into write_row() and similar methods is
actually a pointer to one of these.

Now, the Field objects in table->field are pre-initialised with a pointer
directly into table->record[0]. So you can call them to get data out of
table->record[0] without passing any explicit buffer pointer. You will see
some of the Field methods have a variant that takes a row_offset, so to access
data in another buffer bufptr, you need to pass the pointer difference to
table->record[0]:

    table->field->get_ptr(&my_ptr, bufptr - table->record[0])

which is a bit strange and takes a bit of getting used to.

Hope this helps,

 - Kristian.