Re: update error with serializable

On Fri, Jan 20, 2017 at 4:44 AM, Tom DalPozzo <t(dot)dalpozzo(at)gmail(dot)com> wrote:

> I've two threads countinuously updataing rows in the same table.
> Each one does: BEGIN, UPDATE,UPDATE,,,,COMMIT
> There can't be two active transactions updating the same row (my
> bug apart but I don't think so).
> I'm using default_transaction_isolation = 'serializable'
> I get "could not serialize access due to read/write dependencies
> among transactions"
> I din't expect to see it, hence there must be something in
> postgresql theory that I haven't understood well and I'd like a
> clarification.

Most likely one or both transactions have have updated 3 or more
tuples on a single page, causing the tuple locks for the
transaction on that page to be combined into a single page lock for
that transaction. This is intended to prevent the memory required
for tracking predicate locks from growing too large. This
threshold of 3 per page was entirely arbitrary and always seen as
something which could and should be improved someday. That might
happen for version 10 (expected to be released next year), since a
patch has been submitted to make that configurable.

https://www.postgresql.org/message-id/flat/d8joa0eh9yw(dot)fsf(at)dalvik(dot)ping(dot)uio(dot)no#d8joa0eh9yw(dot)fsf@dalvik.ping.uio.no

If you are able to build from source, you might want to test the
efficacy of the patch for your situation.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company