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

On Wed, Sep 14, 2016 at 12:17 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> For instance, one idea to grow memory usage incrementally would be to
> store dead tuple information separately for each 1GB segment of the
> relation.  So we have an array of dead-tuple-representation objects,
> one for every 1GB of the relation.  If there are no dead tuples in a
> given 1GB segment, then this pointer can just be NULL.  Otherwise, it
> can point to either the bitmap representation (which will take ~4.5MB)
> or it can point to an array of TIDs (which will take 6 bytes/TID).
> That could handle an awfully wide variety of usage patterns
> efficiently; it's basically never worse than what we're doing today,
> and when the dead tuple density is high for any portion of the
> relation it's a lot better.

If you compress the list into a bitmap a posteriori, you know the
number of tuples per page, so you could encode the bitmap even more
efficiently.

It's not a bad idea, one that can be slapped on top of the multiarray
patch - when closing a segment, it can be decided whether to turn it
into a bitmap or not.