Re: [WIP PATCH] Lazily assign xids for toplevel Transactions

"Florian G. Pflug" <fgp(at)phlo(dot)org> writes:
> Sounds good, if we decide to go with the transient XID idea. So below
> for an alternative that I just came up with.

This proposal appears to require taking and releasing a brand-new lock
type every time a snapshot is made or destroyed. That is certainly not
going to be less overhead than the transient-XID scheme. At least in
READ COMMITTED mode, there are normally multiple snapshots taken per
transaction.

(Something worth noting here is that I expect soon, probably 8.4,
we will fix things so that what a backend advertises in MyProc->xmin
is the xmin of its oldest still-live snapshot. That means that xmin
will change intra-transaction in READ COMMITTED mode, and thus that
we would indeed need to take and release the sort of lock you are
suggesting each time.)

regards, tom lane