Re: [RFC] GSoC Work on readonly queries done so far

Jeff Davis wrote:
> On Wed, 2007-06-06 at 22:36 +0100, Simon Riggs wrote:
>>> .) Transactions are assigned a dummy xid ReadOnlyTransactionId, that
>>> is considered to be "later" than any other xid.
>> So you are bumping FirstNormalTransactionId up by one for this?
>>
>> You're assuming then that we will "freeze" replay while we run a query?
>> Otherwise doing this will mean the snapshot changes as a query executes.
>
> Is it possible to put a normal xmax for the snapshot?
>
> It wouldn't be a real transaction on the slave, and also the master will
> use that ID for a real transaction itself. However, I don't see a real
> problem on the slave because it would only be used for the purpose of
> the snapshot we need at that moment.

My plan is the following:
.) Initially, queries and recovery will run interleaved, but not concurrently.
For that, an "empty" snapshot is sufficient, with
xmin=xid=xmax=ReadOnlyTransactionId.
.) Then, I'll work on running them concurrently. The replay process will publish
a "current" snapshot in shared memory, using "real" xmin and xmax values
it generates by maintaining a list of currently active (as in: running when
the wal was written on the master) transactions. In that case, only xid
is set to ReadOnlyTransactionId.

greetings, Florian Pflug