怎么掌握PostgreSQL Locks的基礎(chǔ)知識(shí)

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如下例所示,session 1執(zhí)行update語句,session 2 update相同的rows,session 3查詢locktype為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/掛起

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,進(jìn)程2475中的事務(wù)669310和進(jìn)程2480中的669312分別持有transactionid的ExclusiveLock,進(jìn)程2480在等待事務(wù)ID=669310的lock(granted=f).
為什么會(huì)等待669310的ShareLock呢?回過頭來查看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.
回滾事務(wù)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被更新為669312.

感謝各位的閱讀，以上就是“怎么掌握PostgreSQL Locks的基礎(chǔ)知識(shí)”的內(nèi)容了，經(jīng)過本文的學(xué)習(xí)后，相信大家對(duì)怎么掌握PostgreSQL Locks的基礎(chǔ)知識(shí)這一問題有了更深刻的體會(huì)，具體使用情況還需要大家實(shí)踐驗(yàn)證。這里是億速云，小編將為大家推送更多相關(guān)知識(shí)點(diǎn)的文章，歡迎關(guān)注！