On 23 December 2012 17:38, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
>> The lack of any space saving for lower % values is strange and
>> somewhat worrying. There should be a 36? byte saving for 300 null
>> columns in an 800 column table - how does that not show up at all?
>
> You could only fit about 4 such rows in an 8K page (assuming the columns
> are all int4s). Unless the savings is enough to allow 5 rows to fit in
> a page, the effective savings will be zilch.
If that's the case, the use case is tiny, especially considering how
sensitive the saving is to the exact location of the NULLs.
> This may well mean that the whole thing is a waste of time in most
> scenarios --- the more likely it is to save anything, the more likely
> that the savings will be lost anyway due to page alignment
> considerations, because wider rows inherently pack less efficiently.
ISTM that we'd get a better gain and a wider use case by compressing
the whole block, with some bits masked out to allow updates/deletes.
The string of zeroes in the null bitmap would compress easily, but so
would other aspects also.
-- Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services
