怎么掌握PostgreSQL Locks的基础知识

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如下例所示,session 1执行update语句,session 2 update相同的rows,session 3查询locktype为transactionid的信息.
session 1

[local]:5432 pg12@testdb=# begin; ere relation=295053; BEGIN Time: 1.430 ms [local]:5432 pg12@testdb=#* -- SELECT * from t_lock where id < 10 FOR UPDATE; [local]:5432 pg12@testdb=#* select pg_backend_pid();  pg_backend_pid  ----------------            2475 (1 row) Time: 2.619 ms [local]:5432 pg12@testdb=#* update t_lock set id = 3000 where id = 3; UPDATE 4 Time: 7.892 ms [local]:5432 pg12@testdb=#* select pid,locktype,relation::regclass,mode,page,tuple,virtualxid,transactionid,virtualtransaction,granted,fastpath from pg_locks where relation=295053; -[ RECORD 1 ]------+----------------- pid                | 2475 locktype           | relation relation           | t_lock mode               | RowExclusiveLock page               |  tuple              |  virtualxid         |  transactionid      |  virtualtransaction | 3/2 granted            | t fastpath           | t Time: 9.013 ms

session 2

[local]:5432 pg12@testdb=# ---- session 2 [local]:5432 pg12@testdb=# begin; BEGIN Time: 1.117 ms [local]:5432 pg12@testdb=#* select pg_backend_pid();  pg_backend_pid  ----------------            2480 (1 row) Time: 1.825 ms [local]:5432 pg12@testdb=#* update t_lock set id = 3000 where id = 3; -- 阻塞/挂起

session 3

[local]:5432 pg12@testdb=# select * from pg_locks where pid <> pg_backend_pid() and locktype = 'transactionid';    locktype    | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid | virtualtransaction | pid  |     mode      | granted | fastpath  ---------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+------+---------------+---------+----------  transactionid |          |          |      |       |            |        669310 |         |       |          | 3/2                | 2475 | ExclusiveLock | t       | f  transactionid |          |          |      |       |            |        669312 |         |       |          | 4/4                | 2480 | ExclusiveLock | t       | f  transactionid |          |          |      |       |            |        669310 |         |       |          | 4/4                | 2480 | ShareLock     | f       | f (3 rows) Time: 1.243 ms

可以看到,进程2475中的事务669310和进程2480中的669312分别持有transactionid的ExclusiveLock,进程2480在等待事务ID=669310的lock(granted=f).
为什么会等待669310的ShareLock呢?回过头来查看t_lock表的xmax信息:

[local]:5432 pg12@testdb=# select xmin,xmax,ctid from t_lock where id = 3;   xmin  |  xmax  |  ctid    --------+--------+---------  669246 | 669310 | (0,3)  669247 | 669310 | (4,99)  669248 | 669310 | (8,195)  669252 | 669310 | (13,65) (4 rows) Time: 4.715 ms

可以看到 : 待更新的tuple.xmax = 669310.
回滚事务669310,再次查看xmax:

[local]:5432 pg12@testdb=# select xmin,xmax,ctid from t_lock where id = 3;   xmin  |  xmax  |  ctid    --------+--------+---------  669246 | 669312 | (0,3)  669247 | 669312 | (4,99)  669248 | 669312 | (8,195)  669252 | 669312 | (13,65) (4 rows) Time: 1.182 ms [local]:5432 pg12@testdb=# SELECT pid,backend_xid,wait_event_type,wait_event,state,query FROM pg_stat_activity WHERE pid IN (2475,2480); -[ RECORD 1 ]---+------------------------------------------ pid             | 2475 backend_xid     |  wait_event_type | Client wait_event      | ClientRead state           | idle query           | rollback; -[ RECORD 2 ]---+------------------------------------------ pid             | 2480 backend_xid     | 669312 wait_event_type | Client wait_event      | ClientRead state           | idle in transaction query           | update t_lock set id = 3000 where id = 3; Time: 5.434 ms

xmax被更新为669312.

感谢各位的阅读，以上就是“怎么掌握PostgreSQL Locks的基础知识”的内容了，经过本文的学习后，相信大家对怎么掌握PostgreSQL Locks的基础知识这一问题有了更深刻的体会，具体使用情况还需要大家实践验证。这里是亿速云，小编将为大家推送更多相关知识点的文章，欢迎关注！