Messages, like bellow, showed up in logs couple times:
19538:20121217:131624.429 [Z3005] query failed:  PGRES_FATAL_ERROR:ERROR: deadlock detected
DETAIL: Process 25600 waits for ShareLock on transaction 7119349; blocked by process 25682.
Process 25682 waits for ShareLock on transaction 7119406; blocked by process 25600.
HINT: See server log for query details.
[update items set lastclock=1355734991,lastns=195977702,prevvalue=lastvalue,lastvalue='1' where itemid=23291;
Postgres logs in the attachment.
The 25682 served a pooler process, the 25600 served a history syncer.
The server version is not last, but I didn't find same closed bugs in release notes.
ZBX-5225 Frequent "Lock wait timeout exceeded; try restarting transaction" on zabbix_server. LLD related ?