Re: HOT for PostgreSQL 8.3

mark(at)mark(dot)mielke(dot)cc wrote:
> On Mon, Feb 12, 2007 at 12:48:06AM -0500, Tom Lane wrote:
>> We just finished sweating blood to get the tuple header size down to 23
>> bytes from 27 (which saves 8 bytes not 4 if MAXALIGN=8). We are not
>> going to blow that again on HOT.
>
> I haven't had enough time to follow all of the details here - but if the
> ability to update a row with minimal overhead, as long as there is extra
> room in the same block is a great idea (it sounds appealing to me) - could
> it be done with just a 1 byte list? 24 instead of 23 for the tuple size.

Assuming 8k pages, you could in theory store reference to a line pointer
in just 1 byte.

But actually that 1 free byte in the header is not currently just waste
of space. If you have any nulls in your tuple, there's going to be a
null bitmap in addition to the header. 1 byte is conveniently enough to
store the null bitmap for a table with max 8 columns, and if a table has
more than 8 columns, the extra 4 or 8 bytes needed for the null bitmap
probably doesn't matter so much because the tuples are quite wide anyway.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com