postgresql – postgres中的简单更新查询死锁

根据日志,由于同时执行两个相同的更新而发生死锁.

update public.vm_action_info set last_on_demand_task_id=$1,version=version+1

两个相同的简单更新如何相互死锁？

我在日志中收到的错误

2013-08-18 11:00:24 IDT HINT:  See server log for query details.
2013-08-18 11:00:24 IDT STATEMENT:  update public.vm_action_info set last_on_demand_task_id=$1,version=version+1 where id=$2
2013-08-18 11:00:25 IDT ERROR:  deadlock detected
2013-08-18 11:00:25 IDT DETAIL:  Process 31533 waits for ShareLock on transaction 4228275; blocked by process 31530.
        Process 31530 waits for ExclusiveLock on tuple (0,68) of relation 70337 of database 69205; blocked by process 31533.
        Process 31533: update public.vm_action_info set last_on_demand_task_id=$1,version=version+1 where id=$2
        Process 31530: update public.vm_action_info set last_on_demand_task_id=$1,version=version+1 where id=$2
2013-08-18 11:00:25 IDT HINT:  See server log for query details.
2013-08-18 11:00:25 IDT STATEMENT:  update public.vm_action_info set last_on_demand_task_id=$1,version=version+1 where id=$2
2013-08-18 11:00:25 IDT ERROR:  deadlock detected
2013-08-18 11:00:25 IDT DETAIL:  Process 31530 waits for ExclusiveLock on tuple (0,68) of relation 70337 of database 69205; blocked by process 31876.
        Process 31876 waits for ShareLock on transaction 4228275; blocked by process 31530.
        Process 31530: update public.vm_action_info set last_on_demand_task_id=$1,version=version+1 where id=$2
        Process 31876: update public.vm_action_info set last_on_demand_task_id=$1,version=version+1 where id=$2

模式是：

CREATE TABLE vm_action_info(
  id integer NOT NULL,version integer NOT NULL DEFAULT 0,vm_info_id integer NOT NULL,last_exit_code integer,bundle_action_id integer NOT NULL,last_result_change_time numeric NOT NULL,last_completed_vm_task_id integer,last_on_demand_task_id bigint,CONSTRAINT vm_action_info_pkey PRIMARY KEY (id ),CONSTRAINT vm_action_info_bundle_action_id_fk FOREIGN KEY (bundle_action_id)
      REFERENCES bundle_action (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE CASCADE,CONSTRAINT vm_discovery_info_fk FOREIGN KEY (vm_info_id)
      REFERENCES vm_info (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE CASCADE,CONSTRAINT vm_task_last_on_demand_task_fk FOREIGN KEY (last_on_demand_task_id)
      REFERENCES vm_task (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION,CONSTRAINT vm_task_last_task_fk FOREIGN KEY (last_completed_vm_task_id)
      REFERENCES vm_task (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION
)
WITH (OIDS=FALSE);

ALTER TABLE vm_action_info
  OWNER TO vadm;

-- Index: vm_action_info_vm_info_id_index

-- DROP INDEX vm_action_info_vm_info_id_index;

CREATE INDEX vm_action_info_vm_info_id_index
  ON vm_action_info
  USING btree (vm_info_id );

CREATE TABLE vm_task
(
  id integer NOT NULL,vm_action_info_id integer NOT NULL,creation_time numeric NOT NULL DEFAULT 0,task_state text NOT NULL,triggered_by text NOT NULL,bundle_param_revision bigint NOT NULL DEFAULT 0,execution_time bigint,expiration_time bigint,username text,completion_time bigint,completion_status text,completion_error text,CONSTRAINT vm_task_pkey PRIMARY KEY (id ),CONSTRAINT vm_action_info_fk FOREIGN KEY (vm_action_info_id)
  REFERENCES vm_action_info (id) MATCH SIMPLE
  ON UPDATE NO ACTION ON DELETE CASCADE
)
 WITH (
OIDS=FALSE
);
ALTER TABLE vm_task
  OWNER TO vadm;

-- Index: vm_task_creation_time_index

-- DROP INDEX vm_task_creation_time_index     ;

CREATE INDEX vm_task_creation_time_index
  ON vm_task
  USING btree
 (creation_time );