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, mode
from pg_locks
where pid = pg_backend_pid();
relation | locktype | mode
- ----------+------------+-----------------
pg_locks | relation | AccessShareLock
t1 | 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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