On Tue, Aug 26, 2014 at 11:45 AM, Heikki Linnakangas <hlinnakan...@vmware.com> wrote: >> It appears that this patch weakens the idea of hint bits. Even if >> HEAP_XMIN_COMMITTED is set, it still needs to find out if it's in the >> snapshot. >> >> That's fast if the xid is less than snap->xmin, but otherwise it needs >> to do a fetch from the csnlog, which is exactly what the hint bits are >> designed to avoid. And we can't get around this, because the whole point >> of this patch is to remove the xip array from the snapshot. > > > Yeah. This patch in the current state is likely much much slower than > unpatched master, except in extreme cases where you have thousands of > connections and short transactions so that without the patch, you spend most > of the time acquiring snapshots.
Interesting analysis. I suppose the equivalent of hint bits would be to actually write the CSN of the transaction into the record when the hint bit is set. I don't immediately see how to make that practical. One thought would be to have a list of xids in the page header with their corresponding csn -- which starts to sound a lot like Oralce's "Interested Transaction List". But I don't see how to make that work for the hundreds of possible xids on the page. The worst case for visibility resolution is you have a narrow table that has random access DDL happening all the time, each update is a short transaction and there are a very high rate of such transactions spread out uniformly over a very large table. That means any given page has over 200 rows with random xids spread over a very large range of xids. Currently the invariant hint bits give us is that each xid needs to be looked up in the clog only a more or less fixed number of times, in that scenario only once since the table is very large and the transactions short lived. -- greg -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers