We suffered a deadlock at one point today and I'm looking for some
advice on how to prevent it from happening again. In the postgres log I
found the following:

 

ERROR:  deadlock detected

DETAIL:  Process 20230 waits for ExclusiveLock on relation 31152 of
database 26523; blocked by process 27297.

        Process 27297 waits for AccessExclusiveLock on relation 31205 of
database 26523; blocked by process 20230.

        Process 20230: select "_payments_cluster".ddlScript_prepare(1,
-1);

        Process 27297: select "_payments_cluster".cleanupEvent('10
minutes'::interval, 'false'::boolean);

HINT:  See server log for query details.

CONTEXT:  SQL statement "LOCK TABLE _payments_cluster.sl_event IN
EXCLUSIVE MODE; INSERT INTO _payments_cluster.sl_event (ev_origin,
ev_seqno, ev_timestamp, ev_snapshot, ev_type, ev_data1, ev_data2,
ev_data3, ev_data4, ev_data5, ev_data6, ev_data7, ev_data8) VALUES ('1',
nextval('_payments_cluster.sl_event_seq'), now(),
"pg_catalog".txid_current_snapshot(), $1, $2, $3, $4, $5, $6, $7, $8,
$9); SELECT currval('_payments_cluster.sl_event_seq');"

        SQL statement "SELECT
"_payments_cluster".createEvent('_payments_cluster', 'SYNC', NULL)"

        PL/pgSQL function "ddlscript_prepare" line 29 at PERFORM

STATEMENT:  select "_payments_cluster".ddlScript_prepare(1, -1);

 

So it looks to me like the issue was caused by a ddl script running at
the same time as a slony cleanupEvent command. Our system requires some
dynamic changes to our db structure (in the form of new partitions and
things like that) these ddl scripts could run at any time. So, how can I
best manage this around the cleanupEvents?

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