On 2013-11-21 14:40:36 -0800, Jeff Janes wrote: > But if the transaction would not have otherwise generated WAL (i.e. a > select that did not have to do any HOT pruning, or an update with zero rows > matching the where condition), doesn't it now have to flush and wait when > it would otherwise not?
We short circuit that if there's no xid assigned. Check RecordTransactionCommit().
It looks like that only short-circuits the flush if both there is no xid assigned, and !wrote_xlog. (line 1054 of xact.c)
I do see stalls on fdatasync on flush from select statements which had no xid, but did generate xlog due to HOT pruning, I don't see why WAL logging hint bits would be different.
