Waiting for select

Marc Munro wrote:

Can someone explain this? I seem to have a query which is being blocked
by a lock. I was under the impression that selects are never blocked.
Am I missing something or is this bad behaviour?

Do you happen to be running a vacuum full?

I am using slony and am synchronising a slave for the first time. In
the hope of seeing some progress on the slave I attempt to perform a
select count(*) on one of the tables.

The select just stops. ps shows this:

postgres 5987 0.0 0.4 19180 4188 ? S 15:16 0:00 postgres: postgres testdb 192.168.1.111(33598) SELECT waiting

A query of blocking locks shows this:

object | trans | pid | mode | blocker
---------------------------+-------+------+---------------------+---------
testdb.campaign_cost_pk | | 5754 | AccessShareLock |
testdb.csn_log_pk | | 5754 | RowExclusiveLock |
testdb.pg_trigger | | 5754 | AccessShareLock |
testdb.pg_trigger | | 5754 | RowExclusiveLock |
testdb.sl_subscribe | | 5754 | AccessShareLock |
testdb.csn_log_idx2 | | 5754 | RowExclusiveLock |
testdb.campaign | | 5754 | AccessShareLock |
testdb.campaign | | 5754 | RowExclusiveLock |
testdb.campaign | | 5754 | AccessExclusiveLock |
testdb.pg_rewrite | | 5754 | AccessShareLock |
testdb.pg_rewrite | | 5754 | RowExclusiveLock |
testdb.computer_sn_log | | 5754 | AccessShareLock |
testdb.computer_sn_log | | 5754 | RowExclusiveLock |
testdb.computer_sn_log | | 5754 | AccessExclusiveLock |
testdb.address_type_pk | | 5754 | AccessShareLock |
testdb.campaign_pk | | 5754 | AccessShareLock |
testdb.sl_log_1 | | 5754 | AccessShareLock |
testdb.sl_log_1 | | 5754 | RowExclusiveLock |
testdb.csn_cookie_idx1 | | 5754 | AccessShareLock |
testdb.pg_index | | 5754 | AccessShareLock |
testdb.pg_index | | 5754 | RowShareLock |
testdb.csn_log_idx3 | | 5754 | RowExclusiveLock |
testdb.csn_cookie_pk | | 5754 | AccessShareLock |
testdb.sl_log_2 | | 5754 | AccessShareLock |
testdb.sl_log_2 | | 5754 | RowExclusiveLock |
testdb.sl_set | | 5754 | AccessShareLock |
testdb.sl_set | | 5754 | RowShareLock |
testdb.campaign_cost | | 5754 | AccessShareLock |
testdb.campaign_cost | | 5754 | RowExclusiveLock |
testdb.campaign_cost | | 5754 | AccessExclusiveLock |
testdb.sl_table | | 5754 | AccessShareLock |
testdb.sl_table | | 5754 | RowShareLock |
testdb.sl_table | | 5754 | RowExclusiveLock |
| 9182 | 5754 | ExclusiveLock |
testdb.computer_sn_cookie | | 5754 | AccessShareLock |
testdb.computer_sn_cookie | | 5754 | RowExclusiveLock |
testdb.computer_sn_cookie | | 5754 | AccessExclusiveLock |
testdb.pg_attribute | | 5754 | AccessShareLock |
testdb.sl_config_lock | | 5754 | AccessExclusiveLock |
testdb.sl_trigger | | 5754 | AccessShareLock |
testdb.csn_pk | | 5754 | AccessShareLock |
testdb.pg_class | | 5754 | AccessShareLock |
testdb.pg_class | | 5754 | RowShareLock |
testdb.pg_class | | 5754 | RowExclusiveLock |
testdb.address_type | | 5754 | AccessShareLock |
testdb.address_type | | 5754 | RowExclusiveLock |
testdb.address_type | | 5754 | AccessExclusiveLock |
testdb.pg_namespace | | 5754 | AccessShareLock |
testdb.pg_namespace | | 5754 | RowShareLock |
testdb.csn_log_idx1 | | 5754 | RowExclusiveLock |
testdb.computer_sn | | 5754 | AccessShareLock |
testdb.computer_sn | | 5754 | RowExclusiveLock |
testdb.computer_sn | | 5754 | AccessExclusiveLock |
testdb.address_type | | 5987 | AccessShareLock | 5754
| 9422 | 5987 | ExclusiveLock | 5754
(55 rows)

All responses will be welcomed.

__
Marc

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Marc Munro <marc@bloodnok.com> writes:

Can someone explain this? I seem to have a query which is being blocked
by a lock. I was under the impression that selects are never blocked.

AccessExclusiveLock blocks anything.

A query of blocking locks shows this:

object | trans | pid | mode | blocker
---------------------------+-------+------+---------------------+---------
testdb.address_type | | 5754 | AccessExclusiveLock |
testdb.address_type | | 5987 | AccessShareLock | 5754

So what's process 5754 doing?

regards, tom lane

It's doing something in slony. Part of the initial sync operation I
guess. I guess it must be doing an alter table or reindex or something.
I don't understand why though.

I'll repeat my question on the slony mailing list. Thanks for the
response.

__
Marc