Thread: Why is AccessShareLock held until end of transaction?

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I am probably missing something obvious, but why does the
AccessShareLock remain held on a table after a SELECT statement is
complete when in a transaction block? E.g.:

8<-------------------------
create table t1 ();
begin;
select * from t1;
select relation::regclass, locktype, modefrom pg_lockswhere pid = pg_backend_pid();relation |  locktype  |      mode
- ----------+------------+-----------------pg_locks | relation   | AccessShareLockt1       | relation   |
AccessShareLock        | virtualxid | ExclusiveLock
 
(3 rows)
8<-------------------------

The reason I ask is that I ran into a deadlock situation which was
caused by one session running two SELECT statements in a transaction,
while a second session attempted to create a new table with foreign
keys to two of the tables involved in the first session:

8<-------------------------
- -- at some earlier point
create table t1(id int primary key);
create table t2(id int primary key);

- -- in session 1
begin;
select * from t1;
<idle or race>

- -- in session 2
create table t3
( id int, t2id int references t2(id), t1id int references t1(id)
);
<will block>

- -- in session 1
select * from t2;
<deadlock detected error>
8<-------------------------

Thoughts?

Thanks,

Joe

- -- 
Joe Conway
credativ LLC: http://www.credativ.us
Linux, PostgreSQL, and general Open Source
Training, Service, Consulting, & 24x7 Support
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Joe Conway <mail@joeconway.com> writes:
> I am probably missing something obvious, but why does the
> AccessShareLock remain held on a table after a SELECT statement is
> complete when in a transaction block?

*Any* lock acquired by user command is held till end of transaction;
AccessShareLock isn't special.

In general, releasing early would increase the risk of undesirable
behaviors such as tables changing definition mid-transaction.
        regards, tom lane

On 11 March 2014 03:41, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Joe Conway <mail@joeconway.com> writes:
>> I am probably missing something obvious, but why does the
>> AccessShareLock remain held on a table after a SELECT statement is
>> complete when in a transaction block?
>
> *Any* lock acquired by user command is held till end of transaction;
> AccessShareLock isn't special.
>
> In general, releasing early would increase the risk of undesirable
> behaviors such as tables changing definition mid-transaction.

I thought "good question" at first, but the workaround is simple...
just don't use multi-step transactions, submit each request as a
separate transaction.

-- Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services

On 11 March 2014 17:29, Atri Sharma <atri.jiit@gmail.com> wrote:
>
>
>
> On Tue, Mar 11, 2014 at 10:56 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>>
>> On 11 March 2014 03:41, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> > Joe Conway <mail@joeconway.com> writes:
>> >> I am probably missing something obvious, but why does the
>> >> AccessShareLock remain held on a table after a SELECT statement is
>> >> complete when in a transaction block?
>> >
>> > *Any* lock acquired by user command is held till end of transaction;
>> > AccessShareLock isn't special.
>> >
>> > In general, releasing early would increase the risk of undesirable
>> > behaviors such as tables changing definition mid-transaction.
>>
>> I thought "good question" at first, but the workaround is simple...
>> just don't use multi-step transactions, submit each request as a
>> separate transaction.
>>
>>
> Wouldnt that tend to get inefficient?

Please outline your alternate proposal so we can judge the comparative
efficiency.

-- Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services

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On 03/11/2014 12:26 PM, Simon Riggs wrote:
> On 11 March 2014 03:41, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Joe Conway <mail@joeconway.com> writes:
>>> I am probably missing something obvious, but why does the 
>>> AccessShareLock remain held on a table after a SELECT statement
>>> is complete when in a transaction block?
>> 
>> *Any* lock acquired by user command is held till end of
>> transaction; AccessShareLock isn't special.
>> 
>> In general, releasing early would increase the risk of
>> undesirable behaviors such as tables changing definition
>> mid-transaction.
> 
> I thought "good question" at first, but the workaround is
> simple... just don't use multi-step transactions, submit each
> request as a separate transaction.

Yeah, I told them that already. Unfortunately in this environment it
is not an option. It isn't a huge problem, but I did find it
surprising (as did the client) that a purely read-only transaction
could cause a deadlock with a concurrent CREATE TABLE.

It would seem that once the SELECT statement has finished we could
drop the AccessShareLock, but I guess that would open a can of works
that we don't want to contemplate.

Joe


- -- 
Joe Conway
credativ LLC: http://www.credativ.us
Linux, PostgreSQL, and general Open Source
Training, Service, Consulting, & 24x7 Support
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