PostgreSQL特性矩阵解析系列20之SAVEPOINT

ROLLBACK TO SAVEPOINT — 回滚到一个保存点
RELEASE SAVEPOINT — 销毁一个之前定义的保存点

postgres=# create table test(id int);
CREATE TABLE
postgres=# BEGIN;
BEGIN
postgres=#     INSERT INTO test VALUES (1);
INSERT 0 1
postgres=#     SAVEPOINT my_savepoint;
SAVEPOINT
postgres=#     INSERT INTO test VALUES (2);
INSERT 0 1
postgres=#     ROLLBACK TO SAVEPOINT my_savepoint;
ROLLBACK
postgres=#     INSERT INTO test VALUES (3);
INSERT 0 1
postgres=# COMMIT;
COMMIT
postgres=# select * from test;
 id 
----
  1
  3
(2 rows)
事务将插入值 1 和 3，但不会插入 2。

建立并且稍后销毁一个保存点：
postgres=# BEGIN;
BEGIN
postgres=#     INSERT INTO test VALUES (3);
INSERT 0 1
postgres=#     SAVEPOINT my_savepoint;
SAVEPOINT
postgres=#     INSERT INTO test VALUES (4);
INSERT 0 1
postgres=#     RELEASE SAVEPOINT my_savepoint;
RELEASE
postgres=# COMMIT;
COMMIT
postgres=# select * from test;
 id 
----
  3
  4
(2 rows)
事务将插入 3 和 4。
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PostgreSQL的事物系统是一个三层系统。底层实现低级事务和子事务，在其之上是主循环的控制代码，这些代码又实现了用户可见的事务和保存点。
同时，用户可以通过发出SQL命令BEGIN、COMMIT、ROLLBACK、SAVEPOINT、ROLLBACK TO或RELEASE来改变系统的状态。

涉及的方法主要有以下6个
1.BeginTransactionBlock
2.EndTransactionBlock
3.UserAbortTransactionBlock
4.DefineSavepoint
5.RollbackToSavepoint
6.ReleaseSavepoint

源码路径
src/backend/access/transam/xact.c

/*
 * DefineSavepoint
 *    This executes a SAVEPOINT command.
 */
void
DefineSavepoint(const char *name)
{
  TransactionState s = CurrentTransactionState;

  /*
   * Workers synchronize transaction state at the beginning of each parallel
   * operation, so we can't account for new subtransactions after that
   * point.  (Note that this check will certainly error out if s->blockState
   * is TBLOCK_PARALLEL_INPROGRESS, so we can treat that as an invalid case
   * below.)
   */
   ... ...
   }
   
 
 /*
 * ReleaseSavepoint
 *    This executes a RELEASE command.
 *
 * As above, we don't actually do anything here except change blockState.
 */
void
ReleaseSavepoint(const char *name)
{
  TransactionState s = CurrentTransactionState;
  TransactionState target,
        xact;

  /*
   * Workers synchronize transaction state at the beginning of each parallel
   * operation, so we can't account for transaction state change after that
   * point.  (Note that this check will certainly error out if s->blockState
   * is TBLOCK_PARALLEL_INPROGRESS, so we can treat that as an invalid case
   * below.)
   */
   ... ...
   }
   
   
/*
 * RollbackToSavepoint
 *    This executes a ROLLBACK TO <savepoint> command.
 *
 * As above, we don't actually do anything here except change blockState.
 */
void
RollbackToSavepoint(const char *name)
{
  TransactionState s = CurrentTransactionState;
  TransactionState target,
        xact;

  /*
   * Workers synchronize transaction state at the beginning of each parallel
   * operation, so we can't account for transaction state change after that
   * point.  (Note that this check will certainly error out if s->blockState
   * is TBLOCK_PARALLEL_INPROGRESS, so we can treat that as an invalid case
   * below.)
   */
... ...
}
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