Re: Latest version of Hot Standby patch

Simon Riggs wrote:
> On Wed, 2009-01-07 at 22:08 +0000, Simon Riggs wrote:
>> On Wed, 2009-01-07 at 23:56 +0200, Heikki Linnakangas wrote:
>>> Simon Riggs wrote:
>>>> On Wed, 2009-01-07 at 15:43 +0200, Heikki Linnakangas wrote:
>>>>> When there's no xids in the procarray, couldn't we just use
>>>>> latestCompletedXid instead of calling ReadNewTransactionId()?
>>>> latestCompletedXid is protected by ProcArrayLock so not much difference
>>>> between those two.
>>> The big difference is that we're already holding ProcArrayLock. You
>>> could read the value of latestCompletedXid before releasing
>>> ProcArrayLock, and wouldn't need the retry logic.
>> Sounds good to me then. Will rework.
>
> Applies brakes suddenly.
>
> I realise this is subtle trap I almost fell into the first time I coded
> it. The function is retrieving GetRunningTransactionData() and so we are
> interested in the latest running xid, not the latest completed xid. The
> latter is sufficient for snapshots, but the information derived by
> GetRunningTransactionData() is used to maintain UnobservedXids.

If there's no transactions running, latest completed xid is just what we
need. When there is any transactions in procarray, we should take the
max xid of those, as the patch already does.

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