Postgres10高性能开发（6）锁及其检查

锁类型

参考文档：https://www.postgresql.org/docs/10/explicit-locking.html

表级锁

锁名称	引发操作
ACCESS SHARE 访问共享锁	SELECT 任何读取不修改表的查询都将获取本锁
ROW SHARE 行共享锁	SELECT FOR UPDATE SELECT FOR SHARE
ROW EXCLUSIVE 行排他锁	UPDATE DELETE INSERT
SHARE UPDATE EXCLUSIVE 共享更新排他锁	VACUUM（不含FULL）ANALYZE CREATE INDEX CONCURRENTLY CREATE STATISTICS 部分ALTER TABLE
SHARE 共享锁	CREATE INDEX（无CONCURRENTLY）
SHARE ROW EXCLUSIVE 共享行级排它锁	CREATE COLLATION CREATE TRIGGER 部分ALTER TABLE
EXCLUSIVE 排它锁	REFRESH MATERIALIZED VIEW CONCURRENTLY
ACCESS EXCLUSIVE 访问排他锁	DROP TABLE TRUNCATE REINDEX CLUSTER VACUUM FULL REFRESH MATERIALIZED VIEW (不含 CONCURRENTLY)

Requested Lock Mode	Current Lock Mode
	ACCESS SHARE	ROW SHARE	ROW EXCLUSIVE	SHARE UPDATE EXCLUSIVE	SHARE	SHARE ROW EXCLUSIVE	EXCLUSIVE	ACCESS EXCLUSIVE
ACCESS SHARE								X
ROW SHARE							X	X
ROW EXCLUSIVE					X	X	X	X
SHARE UPDATE EXCLUSIVE				X	X	X	X	X
SHARE			X	X		X	X	X
SHARE ROW EXCLUSIVE			X	X	X	X	X	X
EXCLUSIVE		X	X	X	X	X	X	X
ACCESS EXCLUSIVE	X	X	X	X	X	X	X	X

行级锁

锁定查找到行，阻止对行进行某些操作

锁	阻止操作
FOR UPDATE	UPDATE DELETE SELECT FOR UPDATE SELECT FOR NO KEY UPDATE SELECT FOR SHARE SELECT FOR KEY SHARE
FOR NO KEY UPDATE	UPDATE DELETE SELECT FOR UPDATE SELECT FOR NO KEY UPDATE SELECT FOR SHARE
FOR SHARE	UPDATE DELETE SELECT FOR UPDATE SELECT FOR NO KEY UPDATE
FOR KEY SHARE	UPDATE DELETE SELECT FOR UPDATE

Requested Lock Mode	Current Lock Mode
	FOR KEY SHARE	FOR SHARE	FOR NO KEY UPDATE	FOR UPDATE
FOR KEY SHARE				X
FOR SHARE			X	X
FOR NO KEY UPDATE		X	X	X
FOR UPDATE	X	X	X	X

检查和处理锁

参考文档：https://www.postgresql.org/docs/10/view-pg-locks.html

死锁

如果数据库发现死锁，数据库将自动发现并终止死锁

pg_locks

pg_locks视图记录当前进程的锁定情况，主要字段包括

• locktype：锁定类型，包括 relation（表）, extend, page, tuple（元组）, transactionid（事物ID）, virtualxid, object, userlock, or advisory
• database：锁所在数据库ID，关联到 pg_database.oid
• relation：锁所在的表ID，关联到pg_class.oid
• pid：持有锁或等待锁的进程ID
• mode：持有或等待的锁名称
• granted：True表示数已持有，false表示在等待持有

查找当前的锁

select l.mode,granted,l.locktype,l.pid,query,query_start,db.datname,relname,l.*
from pg_locks l LEFT JOIN pg_stat_activity a on a.pid = l.pid
LEFT JOIN pg_database db on oid = database
LEFT JOIN pg_class on oid = relation
where 1=1
--and l.pid=20139
--and database=(select oid from pg_database where datname='test')
--and relation=(select oid from pg_class where relname='report_user')

排查锁定方法

首先查找一直在等待的锁

select l.mode,granted,l.locktype,l.pid,query,query_start,db.datname,relname,l.*
from pg_locks l LEFT JOIN pg_stat_activity a on a.pid = l.pid
LEFT JOIN pg_database db on oid = database
LEFT JOIN pg_class on oid = relation
where granted = false

在根据锁定类型，锁定对象查找当前持有这个锁的PID及SQL，例如

select l.mode,granted,l.locktype,l.pid,query,query_start,db.datname,relname,l.*
from pg_locks l LEFT JOIN pg_stat_activity a on a.pid = l.pid
LEFT JOIN pg_database db on oid = database
LEFT JOIN pg_class on oid = relation
where transactionid=81401737

判断已获取的锁是否与当前锁冲突，如果冲突，可以使用其PID停止该数据库连接以释放锁
例如：PID为31292

select pg_terminate_backend(31292)