Re: HOT for PostgreSQL 8.3

Ühel kenal päeval, E, 2007-02-12 kell 17:23, kirjutas Heikki
Linnakangas:
> 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,

Are we actually doing that ? I.E are null bitmaps really allocated in 1
byte steps nowadays ?

> 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.
>
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