PostgreSQL 9.1或以上版本，提供了真正意义的Serializability Isolation，本节主要介绍了在此隔离级别下禁止的异象。

测试数据

数据表idx，在id列上有索引，数据库默认的隔离级别为可串行化

15:44:16 [local:/data/run/pg12]:5120 pg12@testdb=# \d+ idx
                                         Table "public.idx"
 Column |       Type        | Collation | Nullable | Default | Storage  | Stats
target | Description
--------+-------------------+-----------+----------+---------+----------+-------
-------+-------------
 id     | integer           |           |          |         | plain    |
       |
 c1     | character varying |           |          |         | extended |
       |
Indexes:
    "idx_id" btree (id)
Access method: heap
[pg12@localhost pg122db]$ grep 'isolation' postgresql.conf
default_transaction_isolation = 'SERIALIZABLE'

Write触发

操作序列如下：

时间点	T1	T2	T3
t1	begin;
t2	select * from idx where id = 1;
t3		begin;
t4		select * from idx where id = 10000;
t5			begin;
t6			update idx set c1 = ‘x’ where id = 10000;
t7			commit;
t8		update idx set c1 = ‘x’ where id = 1;

session 2（T2）在执行update操作时会报错

-- session 2
15:46:52 [local:/data/run/pg12]:5120 pg12@testdb=#* update idx set c1 = 'x' where id = 1;
ERROR:  could not serialize access due to read/write dependencies among transactions
DETAIL:  Reason code: Canceled on identification as a pivot, during write.
HINT:  The transaction might succeed if retried.
15:47:10 [local:/data/run/pg12]:5120 pg12@testdb=#!

Read触发

操作序列如下：

时间点	T1	T2	T3
t1	begin;
t2	select * from idx where id = 1;
t3		begin;
t4		update idx set c1 = ‘x1’ where id = 1;
t5			begin;
t6			update idx set c1 = ‘x’ where id = 10000;
t7			commit;
t8		select * from idx where id = 10000;

session 2（T2）在执行select操作时会报错

15:54:41 [local:/data/run/pg12]:5120 pg12@testdb=#* select * from idx where id = 10000;
ERROR:  could not serialize access due to read/write dependencies among transactions
DETAIL:  Reason code: Canceled on conflict out to pivot 423284, during read.
HINT:  The transaction might succeed if retried.
15:55:16 [local:/data/run/pg12]:5120 pg12@testdb=#!

commit触发

操作序列如下：

时间点	T1	T2
t1	begin;
t2	select * from idx where id = 1;
t3	update tbl set c1 = ‘x’ where id = 10000;
t4		begin;
t5		select * from idx where id = 10000;
t6		update idx set c1 = ‘x’ where id = 1;
t7		commit;
t8	commit;

T1执行commit的时候会报错：

16:07:50 [local:/data/run/pg12]:5120 pg12@testdb=#* commit;
ERROR:  could not serialize access due to read/write dependencies among transactions
DETAIL:  Reason code: Canceled on identification as a pivot, during commit attempt.
HINT:  The transaction might succeed if retried.

为何会触发报错，下节再介绍

PostgreSQL DBA(178) - Serializability Isolation（禁止的异象）

测试数据

Write触发

Read触发

commit触发