Re: Usage of epoch in txid_current

Hi,

On 2017-12-06 17:39:09 +0530, Amit Kapila wrote:
> We are using ShmemVariableCache->nextXid at many places so always
> converting/truncating it to 32-bit number before using seems slightly
> awkward, so we can think of using a separate nextBigXid 64bit number
> as well.

-many

It's not actually that many places. And a lot of them would should be
updated anyway, to be epoch aware. Let's not add this kind of crummyness
to avoid a few truncating casts here and there.

> Either way, it is not clear to me how we will keep it
> updated after recovery. Right now, the mechanism is quite simple, at
> the beginning of a recovery we take the value of nextXid from
> checkpoint record and then if any xlog record indicates xid that
> follows nextXid, we advance it. Here, the point to note is that we
> take the xid from the WAL record (which means that it assumes xids are
> non-contiguous or some xids are consumed without being logged) and
> increment it. Unless we plan to change something in that logic (like
> storing 64-bit xids in WAL records), it is not clear to me how to make
> it work. OTOH, recovering value of epoch which increments only at
> wraparound seems fairly straightforward as described in my previous
> email.

I think it should be fairly simple if simply add the 64bit xid to the
existing clog extension WAL records.

Greetings,

Andres Freund