On 16 August 2016 at 20:58, Greg Stark <stark@mit.edu> wrote:
On Tue, Aug 16, 2016 at 10:15 AM, Craig Ringer <craig@2ndquadrant.com> wrote: > I'm surprised the 32-bit xid was ever exposed to the user, rather than a > 64-bit epoch-extended xid.
Once upon a time we didn't have epoch counting at all.
Makes sense. I didn't dig back too far in history.
Sounds like you're in favour of the 2nd part of the proposal (not covered by the current patch) then.
I haven't yet done the validation required on the epoch logic btw, and I won't be too surprised if it's a bit off. I'm writing a fast xid burn function for use in testing now. I doubt it'll be fast enough to use in routine regression testing since all those clog pages will still take time. But we'll see. I'd kind of like to be able to avoid all that - advance the xid counter and treat all the old xids as frozen. I don't know or if this is practical within a normal backend though.
Anyway, will follow up with more tests and - probably - a bugfix or three soon.
I've written a function to fast-forward the xid counter efficiently, so I can reach xid wraparound in 5s or so. Probably not quite fast enough to be desirable in the basic 'make check' but close. Coming soon.
In the process I noticed that even in the regression tests there are mistakes with xid handling, like
where virtualtransaction = (
select virtualtransaction
from pg_locks
where transactionid = txid_current()::integer)
which breaks if txid_current() returns anything > INT32_MAX.
To do it right(ish) you have to
where virtualtransaction = (
select virtualtransaction
from pg_locks
where transactionid::text::bigint = (txid_current() & (BIGINT '1' << 32)) )
... I think.
So yeah, we need a function to get the 'xid' component from an xid with epoch and/or to fix up things that expose 'xid' to expose bigint txids. The patch on the start of this mail is the first step.
