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
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