Re: Vacuum: allow usage of more than 1GB of work mem

On Thu, Sep 8, 2016 at 11:54 AM, Pavan Deolasee
<pavan(dot)deolasee(at)gmail(dot)com> wrote:
> For example, for a table with 60 bytes wide tuple (including 24 byte
> header), each page can approximately have 8192/60 = 136 tuples. Say we
> provision for 136*2 = 272 bits per page i.e. 34 bytes per page for the
> bitmap. First 272 offsets in every page are represented in the bitmap and
> anything greater than are in overflow region. On the other hand, the current
> representation will need about 16 bytes per page assuming 2% dead tuples, 40
> bytes per page assuming 5% dead tuples and 80 bytes assuming 10% dead
> tuples. So bitmap will take more space for small tuples or when vacuum is
> run very aggressively, both seems unlikely for very large tables. Of course
> the calculation does not take into account the space needed by the overflow
> area, but I expect that too be small.

I thought about something like this, but it could be extremely
inefficient for mostly frozen tables, since the bitmap cannot account
for frozen pages without losing the O(1) lookup characteristic