Wraparound limits

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?

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...

Thank you.

--
Teodor Sigaev E-mail: teodor(at)sigaev(dot)ru
WWW: http://www.sigaev.ru/