On 01/20/2017 10:05 AM, Kevin Grittner wrote:
On Fri, Jan 20, 2017 at 4:44 AM, Tom DalPozzo <t.dalpozzo@xxxxxxxxx> 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.fsf@xxxxxxxxxxxxxxxxxx#d8joa0eh9yw.fsf@xxxxxxxxxxxxxxxxxx
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
Configurable or dynamic? Wouldn't something related to tuples per page
(and maybe fillfactor) do the trick?
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