8.1.2 select for update issue
We're seeing some unexpected behavior in one particular 64-bit
Pgsql 8.1.2 running on HP-UX 11.23 and Itanium 2, built
with --enable-thread-safety. We think we are seeing concurrent
select-for-updates of the same rows by multiple concurrent
backends, contrary to our understanding of select-for-update
semantics. The rows are selected by each client process as
follows:
SELECT *
from foo
where eventprocessed = 'f'
and inprogress = 'f'
and eventstructure is not NULL
order by key asc
for update
limit 25;
Once the rows are selected, they are then updated within the same
transaction, for example, as follows:
update foo set inprogress = 't' where key in (10169339);
We think the row should be locked, unselectable for update, and
that the update above should remove them from selection in any
subsequent select-for-updates like the one above. However, we
see one backend selecting and locking a set of rows, and while
it presumably has them locked and is chugging through doing
updates like the one above, we see another backend
select-for-update grabbing some of the same rows and performing
updates.
We're unable to reproduce this scenario on demand, but it does
consistently happen about 1/3 of the time on this busy system
whenever we turn on the second process. Any suggestions on
where to hunt?
TIA,
Ed
"Ed L." <pgsql@bluepolka.net> writes:
We're seeing some unexpected behavior in one particular 64-bit
Pgsql 8.1.2 running on HP-UX 11.23 and Itanium 2, built
with --enable-thread-safety. We think we are seeing concurrent
select-for-updates of the same rows by multiple concurrent
backends, contrary to our understanding of select-for-update
semantics.
You really ought to be using something newer than 8.1.2. However
I don't see anything directly related in the release notes.
You do have a transaction block established around this whole process?
Row locks only last as long as the current transaction ...
regards, tom lane
On Monday 06 August 2007 1:22 pm, you wrote:
"Ed L." <pgsql@bluepolka.net> writes:
We're seeing some unexpected behavior in one particular
64-bit Pgsql 8.1.2 running on HP-UX 11.23 and Itanium 2,
built with --enable-thread-safety. We think we are seeing
concurrent select-for-updates of the same rows by multiple
concurrent backends, contrary to our understanding of
select-for-update semantics.You really ought to be using something newer than 8.1.2.
Perhaps. But we have yet to find a way to make major version
upgrades of 100+ GB, 100+ tps databases sufficiently inexpensive
and painless in terms of SAN space, performance costs, and
customer downtime on heavily loaded systems. So we put them off
until there is a clear, directly compelling reason to upgrade.
You do have a transaction block established around this whole
process? Row locks only last as long as the current
transaction ...
Of course. This is grasping at straws, but I was wondering if
perhaps anyone saw anything in this behavior that might suggest
a threadsafe-related anomaly?
TIA.
Ed
"Ed L." <pgsql@bluepolka.net> writes:
On Monday 06 August 2007 1:22 pm, you wrote:
You really ought to be using something newer than 8.1.2.
Perhaps. But we have yet to find a way to make major version
upgrades of 100+ GB,
I did not suggest a major version upgrade.
regards, tom lane
On Monday 06 August 2007 2:11 pm, Tom Lane wrote:
"Ed L." <pgsql@bluepolka.net> writes:
On Monday 06 August 2007 1:22 pm, you wrote:
You really ought to be using something newer than 8.1.2.
Perhaps. But we have yet to find a way to make major
version upgrades of 100+ GB,I did not suggest a major version upgrade.
My mistaken assumption. We are considering an upgrade to 8.1.9.
I see the number of bugfixes between 8.1.2 and 8.1.9 is lengthy.
Ed