On 23 December 2012 17:38, Tom Lane <t...@sss.pgh.pa.us> wrote: > Simon Riggs <si...@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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
