Yesterday I wrote:
* On successful completion, the cutoff XID is stored in
pg_class.relvacuumxid, and pg_database.datvacuumxid is updated
if appropriate. (The minxid columns are now useless, but unless there
is another reason to force initdb before 8.2, I'm inclined to leave them
there and unused. We can remove 'em in 8.3.)
After a closer look I am thinking that maybe we should go ahead and
replace relvacuumxid/relminxid and datvacuumxid/datminxid by single
columns named relfrozenxid and datfrozenxid respectively. The reason
is that our documentation has for a long time recommended
SELECT datname, age(datfrozenxid) FROM pg_database;
as a good way to check for databases approaching wraparound.
(In fact it still does ... apparently Alvaro didn't bother to update
maintenance.sgml when he redid that code.)
I don't know how many people might have such queries embedded in
maintenance scripts, but any who do will find their scripts broken by
8.2 as it now stands. Which is a bit silly considering that the
proposed patch will be maintaining a column having exactly the
longstanding definition of datfrozenxid:
All rows inserted by transaction IDs before this one have been
relabeled with a permanent (quotefrozen/) transaction ID in this
database.
I prefer to avoid forcing initdb in late beta, because it causes extra
work for our long-suffering beta testers, but at the moment I'm thinking
this is worth fixing now rather than later. Comments?
regards, tom lane
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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
