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Serializability Isolation下有索引与没有索引的区别。

在没有索引的情况下，对relation进行w（写）操作，PG会对整个relation加SIReadLock，因为加锁粒度是Relation级别，因此如果其他session也对这个表进行w操作，那么两个session之间会出现rw依赖循环，其中一个session会被终止。

-- Session 1[local:/data/run/pg12]:5120 pg12@testdb=# show default_transaction_isolation; default_transaction_isolation------------------------------- serializable(1 row)[local:/data/run/pg12]:5120 pg12@testdb=# begin;BEGIN[local:/data/run/pg12]:5120 pg12@testdb=#* select * from tbl where id = 1; id | c1----+---------------------- 1 | x(1 row)

查询锁信息，在relation上加SIReadLock

[local:/data/run/pg12]:5120 pg12@testdb=# select pid,locktype,relation::regclass,page,tuple,transactionid,mode,granted,fastpath from pg_locks where pid = 22365; pid | locktype | relation | page | tuple | transactionid | mode | granted | fastpath-------+------------+----------+------+-------+---------------+-----------------+---------+---------- 22365 | relation | tbl | | | | AccessShareLock | t | t 22365 | virtualxid | | | | | ExclusiveLock | t | t 22365 | relation | tbl | | | | SIReadLock | t | f(3 rows)

-- Session 1[local:/data/run/pg12]:5120 pg12@testdb=#* update tbl set c1='x' where id = 1;UPDATE 1[local:/data/run/pg12]:5120 pg12@testdb=#* commit;COMMIT[local:/data/run/pg12]:5120 pg12@testdb=#-- Session 2[local:/data/run/pg12]:5120 pg12@testdb=# begin;BEGIN[local:/data/run/pg12]:5120 pg12@testdb=#* update tbl set c1 = 'x' where id = 2;UPDATE 1[local:/data/run/pg12]:5120 pg12@testdb=#* commit;ERROR: could not serialize access due to read/write dependencies among transactionsDETAIL: Reason code: Canceled on identification as a pivot, during commit attempt.HINT: The transaction might succeed if retried.[local:/data/run/pg12]:5120 pg12@testdb=#

操作过程如下：

在存在索引的情况下，对relation进行w（写）操作，PG会对page加SIReadLock，只会影响到tuple所在的page。

[local:/data/run/pg12]:5120 pg12@testdb=# create table tbl_index(id int ,c1 varchar);CREATE TABLE [local:/data/run/pg12]:5120 pg12@testdb=# insert into tbl_index select x,x from generate_series(1,100000) x;INSERT 0 100000[local:/data/run/pg12]:5120 pg12@testdb=# create index idx_tbl_index_id on tbl_index(id);CREATE INDEX[local:/data/run/pg12]:5120 pg12@testdb=# select id,ctid from tbl_index where id in(1,20000); id | ctid-------+---------- 1 | (0,1) 20000 | (107,24)(2 rows)

id为1和20000的tuple位于不同的page中，下面对这两条记录进行更新

-- session 1[local:/data/run/pg12]:5120 pg12@testdb=# begin;BEGIN[local:/data/run/pg12]:5120 pg12@testdb=#* update tbl_index set c1='x' where id = 1;UPDATE 1[local:/data/run/pg12]:5120 pg12@testdb=#*-- session 2[local:/data/run/pg12]:5120 pg12@testdb=# begin;BEGIN[local:/data/run/pg12]:5120 pg12@testdb=#* update tbl_index set c1='x' where id = 20000;UPDATE 1[local:/data/run/pg12]:5120 pg12@testdb=#* select pg_backend_pid(); pg_backend_pid---------------- 22425(1 row)[local:/data/run/pg12]:5120 pg12@testdb=#*

锁信息，注意：锁定的page是index的page而不是heap page

[local:/data/run/pg12]:5120 pg12@testdb=# select pid,locktype,relation::regclass,page,tuple,transactionid,mode,granted,fastpath from pg_locks where pid = 22365; pid | locktype | relation | page | tuple | transactionid | mode | granted | fastpath-------+---------------+------------------+------+-------+---------------+------------------+---------+---------- 22365 | relation | idx_tbl_index_id | | | | RowExclusiveLock | t | t 22365 | relation | tbl_index | | | | RowExclusiveLock | t | t 22365 | virtualxid | | | | | ExclusiveLock | t | t 22365 | transactionid | | | | 423265 | ExclusiveLock | t | f 22365 | page | idx_tbl_index_id | 1 | | | SIReadLock | t | f(5 rows)[local:/data/run/pg12]:5120 pg12@testdb=# select pid,locktype,relation::regclass,page,tuple,transactionid,mode,granted,fastpath from pg_locks where pid = 22425; pid | locktype | relation | page | tuple | transactionid | mode | granted | fastpath-------+---------------+------------------+------+-------+---------------+------------------+---------+---------- 22425 | relation | idx_tbl_index_id | | | | RowExclusiveLock | t | t 22425 | relation | tbl_index | | | | RowExclusiveLock | t | t 22425 | virtualxid | | | | | ExclusiveLock | t | t 22425 | transactionid | | | | 423266 | ExclusiveLock | t | f 22425 | page | idx_tbl_index_id | 56 | | | SIReadLock | t | f(5 rows)

提交事务，两个session均成功

-- session 1[local:/data/run/pg12]:5120 pg12@testdb=#* commit;COMMIT-- session 2[local:/data/run/pg12]:5120 pg12@testdb=#* commit;COMMIT