Wraparound limits

On 08/07/2014 01:34 PM, Teodor Sigaev wrote:

Hi!

I have a questions about setting transaction's wraparound limits. Function
SetTransactionIdLimit() in access/transam/varsup.c:

1)
xidWrapLimit = oldest_datfrozenxid + (MaxTransactionId >> 1);
if (xidWrapLimit < FirstNormalTransactionId)
xidWrapLimit += FirstNormalTransactionId;

Isn't it a problem if oldest_datfrozenxid > MaxTransactionId/2?

Don't think so. What problem do you see?

2)
xidStopLimit = xidWrapLimit - 1000000;
if (xidStopLimit < FirstNormalTransactionId)
xidStopLimit -= FirstNormalTransactionId;

xidWarnLimit = xidStopLimit - 10000000;
if (xidWarnLimit < FirstNormalTransactionId)
xidWarnLimit -= FirstNormalTransactionId;

Why does it use '-' instead of '+' if variable < FirstNormalTransactionId? In
this case it is easy to get xidStopLimit > xidWrapLimit or xidWarnLimit >
xidStopLimit...

Remember that the limits are compared with xids using wrap-around aware
functions TransactionIdPrecedes and TransactionidFollows. Not regular <
and >. The "<" checks above are just to check if the XID hit one of the
special TransactionIds, and if so, increase/decrese it to get back to
the normal range.

- Heikki

--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers