Re: GUC for cleanup indexes threshold.

On Tue, Mar 14, 2017 at 2:48 PM, Peter Geoghegan <pg@bowt.ie> wrote:
> I think that that's safe, but it is a little disappointing that it
> does not allow us to skip work in the case that you really had in mind
> when writing the patch. Better than nothing, though, and perhaps still
> a good start. I would like to hear other people's opinions.

Hmm. So, the SQL-callable function txid_current() exports a 64-bit
XID, which includes an "epoch". If PostgreSQL used these 64-bit XIDs
natively, we'd never need to freeze. Obviously we don't do that
because the per-tuple overhead of visibility information is already
high, and that would make it much worse. But, we can easily afford the
extra overhead in a completely dead page. Maybe we can overcome the
_bt_page_recyclable() limitation by being cleverer about how it
determines if recycling is safe -- it can examine epoch, too. This
would also be required in the similar function
vacuumRedirectAndPlaceholder(), which is a part of SP-GiST.

We already have BTPageOpaqueData.btpo, a union whose contained type
varies based on the page being dead. We could just do the same with
some other field in that struct, and then store epoch there. Clearly
nobody really cares about most data that remains on the page. Index
scans just need to be able to land on it to determine that it's dead,
and VACUUM needs to be able to determine whether or not there could
possibly be such an index scan at the time it considers recycling..

Does anyone see a problem with that?

-- 
Peter Geoghegan