Re: Query failed: ERROR: deadlock detected

On Mon, 2005-08-29 at 16:50 +0100, Aldor wrote:
> PostgreSQL 8.0.3: Any idea what this is?
>
> ---
> Warning: pg_query(): Query failed: ERROR:  deadlock detected
> DETAIL:  Process 8835 waits for ShareLock on transaction 193588236;
> blocked by process 8834.
> Process 8834 waits for ShareLock on transaction 193588228; blocked by
> process 8835
> ---
>
> I have never seen such a log entry until now.

You have two transactions locking each other out.

  For example:
    trans A:  begin;
    trans B:  begin;
    trans A:  select * from table_a for update;    -- locks table_a
    trans B:  select * from table_b for update;    -- locks table_b
    trans A:  select * from table_b for update;    -- blocked by trans B's lock
    trans B:  select * from table_a for update;    -- blocked by trans A's lock

Since the two transactions are blocking each other, the locks will never
be released by the transactions that are holding them.  After a preset
interval (about 1 second by default) a deadlock is detected and reported
with this error message and one of the transactions is aborted.

See manual section 12.3.3 in the 8.0 documentation.

--
Oliver Elphick                                          olly@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver
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