Jim C. Nasby wrote:
Did someone come up with a bitmap compression scheme for on-disk bitmap
indexes that would help out here? Some form of compression could make a
big difference in mostly-dead pages.


Yes, there's a pretty nice compression scheme there, but the requirement for random access makes it a bit hard to use in this case.

If nothing else, it would likely be
worth special-casing an entire page being dead, which is a common case
for queue tables. That could be done by making an entry in the page
number array with a special offset value.

That won't work, because someone might add live tuples to the page after the 1st vacuum pass. You could only invoke that special case when there's no room on the page for new tuples, but that's a hack and not as common.

--
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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