postgresql 锁小测验

pg锁介绍

AccessShareLock：是共享读锁，任何只读取表而不修改它的查询都将获得这种锁模式，此时不允许DDL
ExclusiveLock：文档没找到合适的解释，猜测是全局资源排他锁，对应每个事务的资源排他锁
RowShareLock：select for update ,select for share情况下会出现此锁，此时不允许DML和DDL
RowExclusiveLock：dml会产生此锁，此时不允许dml相同记录，也不允许DDL
AccessExclusiveLock： create、ALTER TABLE、DROP TABLE、TRUNCATE、REINDEX、CLUSTER、VACUUM FULL和REFRESH MATERIALIZED VIEW（不带CONCURRENTLY）表会产生此锁。这种模式保证持有者是访问该表的唯一事务。此锁会阻塞查询
ShareUpdateExclusiveLock：由VACUUM（不带FULL）、ANALYZE、CREATE INDEX CONCURRENTLY、CREATE STATISTICS和ALTER TABLE VALIDATE以及其他ALTER TABLE的变体获得此锁，这种模式保护一个表不受并发模式改变和VACUUM运行的影响。
ShareLock：CREATE INDEX (WITHOUT CONCURRENTLY)会获得此锁，这种模式保护一个表不受并发数据改变的影响，即不能修改此表相关的数据和表结构。
ShareRowExclusiveLock：这种模式保护一个表不受并发数据修改所影响，并且是自排他的，这样在一个时刻只能有一个会话持有它。

AccessShareLock

场景一
session 1;
wjz=# begin;
BEGIN
wjz=# select * from test;
id

1
3
6
2
4
5
(6 rows)

session 2;
select a.pid,b.mode,a.query from pg_stat_activity a left join pg_locks b on a.pid=b.pid;
pid | mode | query
-------±----------------±-----------------------------------------------------------------------------------------
14906 | AccessShareLock | select * from test;
14906 | ExclusiveLock | select * from test;

发现查询除了AccessShareLock 外还有ExclusiveLock 。

RowShareLock

场景二
session 1;
wjz=# begin;
BEGIN
wjz=# select * from test for update;
id

1
3
6
2
4
5
(6 rows)

session 2;
postgres=# select a.pid,b.mode,a.query from pg_stat_activity a left join pg_locks b on a.pid=b.pid order by a.pid ;
pid | mode | query

session 1;
wjz=# begin;
BEGIN
wjz=# select * from test for share;
id

1
3
6
2
4
5
(6 rows)
session 2;
postgres=# select a.pid,b.mode,a.query from pg_stat_activity a left join pg_locks b on a.pid=b.pid order by a.pid ;
pid | mode | query

session 3;
wjz=# begin;
BEGIN
wjz=# update test set id=2 where id<3;

在select for share下也是无法DML和DDL这些表，此处也有ExclusiveLock。

RowExclusiveLock

场景三

session 1;
wjz=# begin;
BEGIN
wjz=# update test set id=3 where id<5;
UPDATE 4

session 2;
postgres=# select a.pid,b.mode,a.query from pg_stat_activity a left join pg_locks b on a.pid=b.pid order by a.pid ;
pid | mode | query

session 1;
wjz=# begin;
BEGIN
wjz=# insert into test values(10);
INSERT 0 1
wjz=#

session 2;
postgres=# select a.pid,b.mode,a.query from pg_stat_activity a left join pg_locks b on a.pid=b.pid order by a.pid ;
pid | mode | query

session 1;
wjz=# begin;
BEGIN
wjz=# delete from test where id<2;
DELETE 1

session 2;
postgres=# select a.pid,b.mode,a.query from pg_stat_activity a left join pg_locks b on a.pid=b.pid order by a.pid ;
pid | mode | query

AccessExclusiveLock

场景四

session 1;
wjz=# begin;
BEGIN
wjz=# create table student(name varchar(10));
CREATE TABLE

wjz=# begin;
BEGIN
wjz=# alter table test add column name varchar(20);
ALTER TABLE

postgres=# select a.pid,b.mode,a.query from pg_stat_activity a left join pg_locks b on a.pid=b.pid order by a.pid ;
pid | mode | query

-------±--------------------±------------------------------------------------------------------------------------------

session 3;
wjz=# begin;
BEGIN
wjz=# update test set id=2 where id<3; —出现锁等待

ShareUpdateExclusiveLock

场景五
session 1;
wjz=# CREATE INDEX CONCURRENTLY idx_test1 on test(name) ;
CREATE INDEX

session 2;
postgres=# select a.pid,b.mode,a.query from pg_stat_activity a left join pg_locks b on a.pid=b.pid order by a.pid ;
pid | mode | query

-------±-------------------------±-------------------------------------------------------------------------------------
14906 | RowExclusiveLock | CREATE INDEX CONCURRENTLY idx_test1 on test(name) ;
14906 | RowExclusiveLock | CREATE INDEX CONCURRENTLY idx_test1 on test(name) ;
14906 | RowExclusiveLock | CREATE INDEX CONCURRENTLY idx_test1 on test(name) ;
14906 | ExclusiveLock | CREATE INDEX CONCURRENTLY idx_test1 on test(name) ;
14906 | AccessExclusiveLock | CREATE INDEX CONCURRENTLY idx_test1 on test(name) ;
14906 | ExclusiveLock | CREATE INDEX CONCURRENTLY idx_test1 on test(name) ;
14906 | ShareUpdateExclusiveLock | CREATE INDEX CONCURRENTLY idx_test1 on test(name) ;

session 3;
wjz=# select * from test; —发生等待

ShareRowExclusiveLock

这个锁才疏学浅，没有模拟出来，后续遇到再补充。

官网锁兼容性表格：
postgresql 锁小测验
实验总结：
（1）ExclusiveLock这个锁出现在每个会话中，从字面意思理解是互斥锁，但从实验中发现此锁对表的其他锁不造成冲突，应该是负责管理事务本身资源方面的锁的占用。
（2）AccessShareLock与ShareUpdateExclusiveLock都不阻塞读写，但会阻塞DDL。
（3）AccessExclusiveLock会阻塞读写，但与ShareUpdateExclusiveLock共同存在下，不阻塞读写（具体原因估计和锁的优先级或者加锁顺序有关）。
（4）RowExclusiveLock：一般是DML产生的，但DDL某些情况下也会产生。
postgresql锁的资料网上太少了，测试完还是晕乎乎的，掌握一下锁出问题的解决办法吧！

postgresql 锁小测验

查看等待锁的SQL:
select a.pid,a.usename,a.query,a.wait_event_type,b.mode,now()-a.xact_start as xid_startime from pg_stat_activity a join pg_locks b on a.pid=b.pid where not b.granted order by xid_startime asc;
查看当前数据库执行SQL情况
select datname,pid,usename,application_name,client_addr,now()-xact_start as xid_startime,now()-query_start as query_startime,now()-state_change as state_changetime,wait_event_type,state,query from pg_stat_activity where query !=’’ order by xid_startime asc;
查询阻塞源的函数
select pg_blocking_pids(23224);
取消阻塞源的函数：
select pg_terminate_backend(14906); 可释放阻塞源，这个相当于杀掉会话进程
select PG_CANCEL_BACKEND(14906);取消后台操作，回滚未提交事务，但不会杀掉会话进程

pg锁介绍

AccessShareLock

场景一 session 1; wjz=# begin; BEGIN wjz=# select * from test; id

RowShareLock

场景二 session 1; wjz=# begin; BEGIN wjz=# select * from test for update; id

session 1; wjz=# begin; BEGIN wjz=# select * from test for share; id

RowExclusiveLock

AccessExclusiveLock

-------±--------------------±------------------------------------------------------------------------------------------

ShareUpdateExclusiveLock

ShareRowExclusiveLock

场景一
session 1;
wjz=# begin;
BEGIN
wjz=# select * from test;
id

场景二
session 1;
wjz=# begin;
BEGIN
wjz=# select * from test for update;
id

session 1;
wjz=# begin;
BEGIN
wjz=# select * from test for share;
id