PostgreSQL的学习心得和知识总结(一百四十七)|深入理解PostgreSQL数据库之transaction chain的使用和实现

目录结构

注:提前言明 本文借鉴了以下博主、书籍或网站的内容,其列表如下:

1、参考书籍:++《PostgreSQL数据库内核分析》++

2、参考书籍:++《数据库事务处理的艺术:事务管理与并发控制》++

3、++PostgreSQL数据库仓库链接,点击前往++

4、++日本著名PostgreSQL数据库专家 铃木启修 网站主页,点击前往++

5、参考书籍:++《PostgreSQL中文手册》++

6、++参考书籍:《PostgreSQL指南:内幕探索》,点击前往++

7、++Using Transaction Chaining to Reduce Server Round-Trips,点击前往++


1、本文内容全部来源于开源社区 GitHub和以上博主的贡献,本文也免费开源(可能会存在问题,评论区等待大佬们的指正)

2、本文目的:开源共享 抛砖引玉 一起学习

3、本文不提供任何资源 不存在任何交易 与任何组织和机构无关

4、大家可以根据需要自行 复制粘贴以及作为其他个人用途,但是不允许转载 不允许商用 (写作不易,还请见谅 💖)

5、本文内容基于PostgreSQL master源码开发而成


深入理解PostgreSQL数据库之transaction chain的使用和实现



文章快速说明索引

学习目标:

做数据库内核开发久了就会有一种 少年得志,年少轻狂 的错觉,然鹅细细一品觉得自己其实不算特别优秀 远远没有达到自己想要的。也许光鲜的表面掩盖了空洞的内在,每每想到于此,皆有夜半临渊如履薄冰之感。为了睡上几个踏实觉,即日起 暂缓其他基于PostgreSQL数据库的兼容功能开发,近段时间 将着重于学习分享Postgres的基础知识和实践内幕。


学习内容:(详见目录)

1、深入理解PostgreSQL数据库之transaction chain的使用和实现


学习时间:

2024年07月01日 20:25:11


学习产出:

1、PostgreSQL数据库基础知识回顾 1个
2、CSDN 技术博客 1篇
3、PostgreSQL数据库内核深入学习


注:下面我们所有的学习环境是Centos8+PostgreSQL master+Oracle19C+MySQL8.0

sql 复制代码
postgres=# select version();
                                                  version                                                   
------------------------------------------------------------------------------------------------------------
 PostgreSQL 17devel on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-21), 64-bit
(1 row)

postgres=#

#-----------------------------------------------------------------------------#

SQL> select * from v$version;          

BANNER        Oracle Database 19c EE Extreme Perf Release 19.0.0.0.0 - Production	
BANNER_FULL	  Oracle Database 19c EE Extreme Perf Release 19.0.0.0.0 - Production Version 19.17.0.0.0	
BANNER_LEGACY Oracle Database 19c EE Extreme Perf Release 19.0.0.0.0 - Production	
CON_ID 0


#-----------------------------------------------------------------------------#

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.27    |
+-----------+
1 row in set (0.06 sec)

mysql>

功能使用背景说明

使用 PostgreSQL 或任何关系数据库实现业务应用程序通常相当于执行一系列事务。给定事务以 COMMIT 还是 ROLLBACK 结束并不重要,因为在这两种情况下,下一个事务都会在前一个事务完成后立即开始。此外,对于大多数应用程序来说,前一个事务和下一个事务具有相似的特征,例如它们的 ISOLATION LEVEL。本质上,您最终得到的样式如下所示:

sql 复制代码
START TRANSACTION;
-- workload of 1st transaction
COMMIT;
START TRANSACTION;
-- workload of 2nd transaction
ROLLBACK;
START TRANSACTION;
-- workload of 3rd transaction
COMMIT;

在上面的 SQL 脚本中,您会看到三个后续事务和总共六个语句(每个事务两个语句),但您可以想象更长的事务序列。问题是:每个语句都需要单独的服务器往返才能执行。没有办法解决这个问题,即使三个示例事务为空(不包含任何语句),也没有什么区别。


监控连接状态

为了进行此实验,我们需要与同一 PostgreSQL 服务器建立两个单独的连接(例如,两个 psql 会话)。第一个连接使用 postgres 数据库,第二个连接使用demo数据库。

好了,现在我们可以在第一个连接上使用 pg_stat_activity(属于 pg_catalog 模式的内置视图)来询问 PostgreSQL 使用演示数据库的第二个连接的状态:

sql 复制代码
postgres=# SELECT state FROM pg_stat_activity WHERE datname='demo';
 state
-------
 idle
(1 row)

如您所见,由于没有执行任何操作,因此该连接当前处于空闲状态。但是,如果我们在第二个连接上启动事务,我们可以看到连接的状态立即变为idle in transaction

sql 复制代码
demo=# START TRANSACTION;
START TRANSACTION
sql 复制代码
postgres=# SELECT state FROM pg_stat_activity WHERE datname='demo';
        state
---------------------
 idle in transaction
(1 row)

这证明 START TRANSACTION 语句导致了与服务器的往返,因为如果没有,第一个连接将无法看到第二个连接的状态变化。如果我们使用 COMMITROLLBACK 完成事务,情况也是一样的,在这种情况下,连接会立即恢复到空闲状态:

sql 复制代码
demo=*# COMMIT;
COMMIT
sql 复制代码
postgres=# SELECT state FROM pg_stat_activity WHERE datname='demo';
 state
-------
 idle
(1 row)

往返开销

长话短说,事务的开始和结束可能会带来巨大的开销。具体来说,如果满足以下任何条件:

  1. 往返服务器的时间很慢。通常,如果客户端和服务器之间的网络距离很远,情况就会如此。
  2. 许多事务的平均运行时间很短。这是因为对于较短的事务,开销占总运行时间的百分比较高。

那么,我们可以做些什么来减少开销并提高性能呢?


事务链

SQL 标准有一个由 PostgreSQL 实现的内置解决方案:AND CHAIN 参数。此参数可用于 COMMITROLLBACK 语句,并具有以下效果...

如果提供了 AND CHAIN 参数,则提交(或回滚)当前事务,此外,立即启动具有相同特征(例如,ISOLATION LEVEL)的后续事务。

因此,如果我们将其应用于原始示例,我们可以将服务器往返次数减少基本上 50%(从 n 减少到 n/2+1)。

sql 复制代码
START TRANSACTION;
-- workload of 1st transaction
COMMIT AND CHAIN;
-- workload of 2nd transaction
ROLLBACK AND CHAIN;
-- workload of 3rd transaction
COMMIT;

我们可以运行相同的实验来证明它按预期工作。我再次使用两个连接,一个使用 postgres 数据库,另一个使用演示数据库。最初,演示连接处于空闲状态,但是一旦我们开始新的事务,其状态就会更改为idle in transaction

sql 复制代码
demo=# START TRANSACTION;
START TRANSACTION
sql 复制代码
postgres=# SELECT state FROM pg_stat_activity WHERE datname='demo';
        state
---------------------
 idle in transaction
(1 row)

那么,如果demo连接执行 COMMIT AND CHAIN 语句会发生什么?正如我所说,PostgreSQL 立即启动后续事务,因此我们看不到状态变化。

sql 复制代码
demo=# COMMIT AND CHAIN;
COMMIT AND CHAIN
sql 复制代码
postgres=# SELECT state FROM pg_stat_activity WHERE datname='demo';
        state
---------------------
 idle in transaction
(1 row)

当我们执行 ROLLBACK AND CHAIN 时,我们得到完全相同的行为 --- 没有明显的状态改变。

sql 复制代码
demo=# ROLLBACK AND CHAIN;
ROLLBACK AND CHAIN
sql 复制代码
postgres=# SELECT state FROM pg_stat_activity WHERE datname='demo';
        state
---------------------
 idle in transaction
(1 row)

最后,当我们发出正常的COMMITROLLBACK时,状态就会变回初始的空闲状态。

sql 复制代码
demo=# COMMIT;
COMMIT
sql 复制代码
postgres=# SELECT state FROM pg_stat_activity WHERE datname='demo';
 state
-------
 idle
(1 row)

功能实现源码分析

其语法格式如下:

c 复制代码
// src/backend/parser/gram.y

/*****************************************************************************
 *
 *		Transactions:
 *
 *		BEGIN / COMMIT / ROLLBACK
 *		(also older versions END / ABORT)
 *
 *****************************************************************************/

TransactionStmt:
			ABORT_P opt_transaction opt_transaction_chain
				{
					TransactionStmt *n = makeNode(TransactionStmt);

					n->kind = TRANS_STMT_ROLLBACK;
					n->options = NIL;
					n->chain = $3;
					n->location = -1;
					$$ = (Node *) n;
				}
			...
			| COMMIT opt_transaction opt_transaction_chain
				{
					TransactionStmt *n = makeNode(TransactionStmt);

					n->kind = TRANS_STMT_COMMIT;
					n->options = NIL;
					n->chain = $3;
					n->location = -1;
					$$ = (Node *) n;
				}
			| ROLLBACK opt_transaction opt_transaction_chain
				{
					TransactionStmt *n = makeNode(TransactionStmt);

					n->kind = TRANS_STMT_ROLLBACK;
					n->options = NIL;
					n->chain = $3;
					n->location = -1;
					$$ = (Node *) n;
				}
			...
		;
		
TransactionStmtLegacy:
			...
			| END_P opt_transaction opt_transaction_chain
				{
					TransactionStmt *n = makeNode(TransactionStmt);

					n->kind = TRANS_STMT_COMMIT;
					n->options = NIL;
					n->chain = $3;
					n->location = -1;
					$$ = (Node *) n;
				}
		;

opt_transaction_chain:
			AND CHAIN		{ $$ = true; }
			| AND NO CHAIN	{ $$ = false; }
			| /* EMPTY */	{ $$ = false; }
		;

示例一,如下:



示例二,如下:



源码调试案例分析

接下来,我们调试一下 重点看一下上面的示例二,如下:

如上,begin READ ONLY;是在上图将guc参数transaction_read_only设置为真 XactReadOnly = true,函数堆栈,如下:

c 复制代码
set_config_with_handle(const char * name, config_handle * handle, const char * value, GucContext context, GucSource source, Oid srole, GucAction action, _Bool changeVal, int elevel, _Bool is_reload) (\home\postgres\postgres\src\backend\utils\misc\guc.c:3758)
set_config_option(const char * name, const char * value, GucContext context, GucSource source, GucAction action, _Bool changeVal, int elevel, _Bool is_reload) (\home\postgres\postgres\src\backend\utils\misc\guc.c:3361)
SetPGVariable(const char * name, List * args, _Bool is_local) (\home\postgres\postgres\src\backend\utils\misc\guc_funcs.c:320)
standard_ProcessUtility(PlannedStmt * pstmt, const char * queryString, _Bool readOnlyTree, ProcessUtilityContext context, ParamListInfo params, QueryEnvironment * queryEnv, DestReceiver * dest, QueryCompletion * qc) (\home\postgres\postgres\src\backend\tcop\utility.c:619)
ProcessUtility(PlannedStmt * pstmt, const char * queryString, _Bool readOnlyTree, ProcessUtilityContext context, ParamListInfo params, QueryEnvironment * queryEnv, DestReceiver * dest, QueryCompletion * qc) (\home\postgres\postgres\src\backend\tcop\utility.c:523)
PortalRunUtility(Portal portal, PlannedStmt * pstmt, _Bool isTopLevel, _Bool setHoldSnapshot, DestReceiver * dest, QueryCompletion * qc) (\home\postgres\postgres\src\backend\tcop\pquery.c:1158)
PortalRunMulti(Portal portal, _Bool isTopLevel, _Bool setHoldSnapshot, DestReceiver * dest, DestReceiver * altdest, QueryCompletion * qc) (\home\postgres\postgres\src\backend\tcop\pquery.c:1315)
PortalRun(Portal portal, long count, _Bool isTopLevel, _Bool run_once, DestReceiver * dest, DestReceiver * altdest, QueryCompletion * qc) (\home\postgres\postgres\src\backend\tcop\pquery.c:791)
exec_simple_query(const char * query_string) (\home\postgres\postgres\src\backend\tcop\postgres.c:1274)
PostgresMain(const char * dbname, const char * username) (\home\postgres\postgres\src\backend\tcop\postgres.c:4680)
BackendMain(char * startup_data, size_t startup_data_len) (\home\postgres\postgres\src\backend\tcop\backend_startup.c:105)
postmaster_child_launch(BackendType child_type, char * startup_data, size_t startup_data_len, ClientSocket * client_sock) (\home\postgres\postgres\src\backend\postmaster\launch_backend.c:265)
BackendStartup(ClientSocket * client_sock) (\home\postgres\postgres\src\backend\postmaster\postmaster.c:3593)
ServerLoop() (\home\postgres\postgres\src\backend\postmaster\postmaster.c:1674)
PostmasterMain(int argc, char ** argv) (\home\postgres\postgres\src\backend\postmaster\postmaster.c:1372)
main(int argc, char ** argv) (\home\postgres\postgres\src\backend\main\main.c:197)

接下来,这里将直接INSERT,报错如下:

后续处理,如下:

此时的函数堆栈,如下:

c 复制代码
AtEOXact_GUC(_Bool isCommit, int nestLevel)
AbortTransaction()
AbortCurrentTransactionInternal()
AbortCurrentTransaction()
PostgresMain(const char * dbname, const char * username)
BackendMain(char * startup_data, size_t startup_data_len)
postmaster_child_launch(BackendType child_type, char * startup_data, size_t startup_data_len, ClientSocket * client_sock)
BackendStartup(ClientSocket * client_sock)
ServerLoop()
PostmasterMain(int argc, char ** argv)
main(int argc, char ** argv)

注:如上 在INSERT报错之后,该事务对应的上述GUC被重置,如下:

c 复制代码
// src/backend/utils/misc/guc.c

/*
 * Do GUC processing at transaction or subtransaction commit or abort, or
 * when exiting a function that has proconfig settings, or when undoing a
 * transient assignment to some GUC variables.  (The name is thus a bit of
 * a misnomer; perhaps it should be ExitGUCNestLevel or some such.)
 * During abort, we discard all GUC settings that were applied at nesting
 * levels >= nestLevel.  nestLevel == 1 corresponds to the main transaction.
 *  
 * 在事务或子事务提交或中止时,或在退出具有 proconfig 设置的函数时,或在撤消对某些 GUC 变量的临时分配时,执行 GUC 处理
 * (因此,这个名字有点用词不当;也许应该是 ExitGUCNestLevel 或类似的名字)
 * 在中止期间,我们会丢弃在嵌套级别 >= nestLevel 处应用的所有 GUC 设置
 * nestLevel == 1 对应于主事务
 */
void
AtEOXact_GUC(bool isCommit, int nestLevel);

于是在接下来的commit and chain;中,XactReadOnly仍是假,如下:

此时函数堆栈,如下:

c 复制代码
EndTransactionBlock(_Bool chain)
standard_ProcessUtility(PlannedStmt * pstmt, const char * queryString, _Bool readOnlyTree, ProcessUtilityContext context, ParamListInfo params, QueryEnvironment * queryEnv, DestReceiver * dest, QueryCompletion * qc)
ProcessUtility(PlannedStmt * pstmt, const char * queryString, _Bool readOnlyTree, ProcessUtilityContext context, ParamListInfo params, QueryEnvironment * queryEnv, DestReceiver * dest, QueryCompletion * qc)
PortalRunUtility(Portal portal, PlannedStmt * pstmt, _Bool isTopLevel, _Bool setHoldSnapshot, DestReceiver * dest, QueryCompletion * qc) 
PortalRunMulti(Portal portal, _Bool isTopLevel, _Bool setHoldSnapshot, DestReceiver * dest, DestReceiver * altdest, QueryCompletion * qc)
PortalRun(Portal portal, long count, _Bool isTopLevel, _Bool run_once, DestReceiver * dest, DestReceiver * altdest, QueryCompletion * qc)
exec_simple_query(const char * query_string)
...

接下来,我们调试一下上面的示例二的另一种情况,因为当前会话已经设置该GUC参数为真(将要被rollback或者commit的事务 ),接下来的rollback and chain的处理 如下:

c 复制代码
UserAbortTransactionBlock(_Bool chain) (\home\postgres\postgres\src\backend\access\transam\xact.c:4262)
standard_ProcessUtility(PlannedStmt * pstmt, const char * queryString, _Bool readOnlyTree, ProcessUtilityContext context, ParamListInfo params, QueryEnvironment * queryEnv, DestReceiver * dest, QueryCompletion * qc)
ProcessUtility(PlannedStmt * pstmt, const char * queryString, _Bool readOnlyTree, ProcessUtilityContext context, ParamListInfo params, QueryEnvironment * queryEnv, DestReceiver * dest, QueryCompletion * qc)
PortalRunUtility(Portal portal, PlannedStmt * pstmt, _Bool isTopLevel, _Bool setHoldSnapshot, DestReceiver * dest, QueryCompletion * qc)
PortalRunMulti(Portal portal, _Bool isTopLevel, _Bool setHoldSnapshot, DestReceiver * dest, DestReceiver * altdest, QueryCompletion * qc)
PortalRun(Portal portal, long count, _Bool isTopLevel, _Bool run_once, DestReceiver * dest, DestReceiver * altdest, QueryCompletion * qc)
exec_simple_query(const char * query_string)
...

在UserAbortTransactionBlock函数中的处理,如下:

c 复制代码
s->blockState: TBLOCK_INPROGRESS -> TBLOCK_ABORT_PENDING
s->chain: false -> true

然后进入如下的处理逻辑:

c 复制代码
CommitTransactionCommandInternal()
CommitTransactionCommand()
finish_xact_command()
exec_simple_query(const char * query_string)
...
c 复制代码
			/*
			 * Here we were in a perfectly good transaction block but the user
			 * told us to ROLLBACK anyway.  We have to abort the transaction
			 * and then clean up.
			 */
		case TBLOCK_ABORT_PENDING:
			AbortTransaction();
			CleanupTransaction();
			s->blockState = TBLOCK_DEFAULT;
			if (s->chain)
			{
				StartTransaction();
				s->blockState = TBLOCK_INPROGRESS;
				s->chain = false;
				RestoreTransactionCharacteristics(&savetc);
			}
			break;

首先进入AbortTransaction函数,因为在这种情况下guc_stack_list != NIL,在如下的堆栈处理中 该参数XactReadOnly被置为假:

c 复制代码
AtEOXact_GUC(_Bool isCommit, int nestLevel)
AbortTransaction()
CommitTransactionCommandInternal()
CommitTransactionCommand()
finish_xact_command()
exec_simple_query(const char * query_string)
...

接着进入CleanupTransaction函数进行清理!


接下来进入今天的重点,如下:

c 复制代码
			if (s->chain)
			{
				StartTransaction();
				s->blockState = TBLOCK_INPROGRESS;
				s->chain = false;
				RestoreTransactionCharacteristics(&savetc);
			}

因为chain为真,这里还是再重启一个事务。不过这种类似于XactReadOnly的guc参数在StartTransaction过程中仍被赋值默认值!该参数的重新修改如下:

至此,rollback and chain 操作回滚了上一个事务,并开启新的事务 且XactReadOnly = true,得以保留!


同上面rollback and chain操作一样,其他几种 如下:

c 复制代码
// src/backend/access/transam/xact.c

/*
 *	CommitTransactionCommandInternal - a function doing an iteration of work
 *		regarding handling the commit transaction command.  In the case of
 *		subtransactions more than one iterations could be required.  Returns
 *		true when no more iterations required, false otherwise.
 */
static bool
CommitTransactionCommandInternal(void)
{
	...
			/*
			 * We are completing a "COMMIT" command.  Do it and return to the
			 * idle state.
			 */
		case TBLOCK_END:
			CommitTransaction();
			s->blockState = TBLOCK_DEFAULT;
			if (s->chain)				// here
			{
				StartTransaction();
				s->blockState = TBLOCK_INPROGRESS;
				s->chain = false;
				RestoreTransactionCharacteristics(&savetc);
			}
			break;
	...
			/*
			 * Here we were in an aborted transaction block and we just got
			 * the ROLLBACK command from the user, so clean up the
			 * already-aborted transaction and return to the idle state.
			 */
		case TBLOCK_ABORT_END:
			CleanupTransaction();
			s->blockState = TBLOCK_DEFAULT;
			if (s->chain)				// here
			{
				StartTransaction();
				s->blockState = TBLOCK_INPROGRESS;
				s->chain = false;
				RestoreTransactionCharacteristics(&savetc);
			}
			break;
	...
			/*
			 * Here we were in a perfectly good transaction block but the user
			 * told us to ROLLBACK anyway.  We have to abort the transaction
			 * and then clean up.
			 */
		case TBLOCK_ABORT_PENDING:
			AbortTransaction();
			CleanupTransaction();
			s->blockState = TBLOCK_DEFAULT;
			if (s->chain)				// here
			{
				StartTransaction();
				s->blockState = TBLOCK_INPROGRESS;
				s->chain = false;
				RestoreTransactionCharacteristics(&savetc);
			}
			break;
	...
			/*
			 * The user issued a COMMIT, so we end the current subtransaction
			 * hierarchy and perform final commit. We do this by rolling up
			 * any subtransactions into their parent, which leads to O(N^2)
			 * operations with respect to resource owners - this isn't that
			 * bad until we approach a thousands of savepoints but is
			 * necessary for correctness should after triggers create new
			 * resource owners.
			 */
		case TBLOCK_SUBCOMMIT:
			do
			{
				CommitSubTransaction();
				s = CurrentTransactionState;	/* changed by pop */
			} while (s->blockState == TBLOCK_SUBCOMMIT);
			/* If we had a COMMIT command, finish off the main xact too */
			if (s->blockState == TBLOCK_END)
			{
				Assert(s->parent == NULL);
				CommitTransaction();
				s->blockState = TBLOCK_DEFAULT;
				if (s->chain)				// here
				{
					StartTransaction();
					s->blockState = TBLOCK_INPROGRESS;
					s->chain = false;
					RestoreTransactionCharacteristics(&savetc);
				}
			}
			else if (s->blockState == TBLOCK_PREPARE)
			{
				Assert(s->parent == NULL);
				PrepareTransaction();
				s->blockState = TBLOCK_DEFAULT;
			}
			else
				elog(ERROR, "CommitTransactionCommand: unexpected state %s",
					 BlockStateAsString(s->blockState));
			break;
	...
}

对应非特殊的guc参数,能否可以继承呢?如下:

sql 复制代码
[postgres@localhost:~/test/bin]$ ./psql 
psql (17beta2)
Type "help" for help.

postgres=# show timezone;
      TimeZone       
---------------------
 America/Los_Angeles
(1 row)

postgres=# begin;
BEGIN
postgres=*# set timezone = 'PRC';
SET
postgres=*# show timezone;
 TimeZone 
----------
 PRC
(1 row)

postgres=*# commit and chain; ## commit 提交 && 继承
COMMIT
postgres=*# show timezone;
 TimeZone 
----------
 PRC
(1 row)

postgres=*# rollback and chain; ## 无东西可以回滚
ROLLBACK
postgres=*# show timezone;
 TimeZone 
----------
 PRC
(1 row)

postgres=*#
sql 复制代码
[postgres@localhost:~/test/bin]$ ./psql 
psql (17beta2)
Type "help" for help.

postgres=# show timezone;
      TimeZone       
---------------------
 America/Los_Angeles
(1 row)

postgres=# begin;
BEGIN
postgres=*# desc a error;
2024-07-01 06:19:56.969 PDT [34810] ERROR:  syntax error at or near "desc" at character 1
2024-07-01 06:19:56.969 PDT [34810] STATEMENT:  desc a error;
ERROR:  syntax error at or near "desc"
LINE 1: desc a error;
        ^
postgres=!# show timezone;
2024-07-01 06:20:00.721 PDT [34810] ERROR:  current transaction is aborted, commands ignored until end of transaction block
2024-07-01 06:20:00.721 PDT [34810] STATEMENT:  show timezone;
ERROR:  current transaction is aborted, commands ignored until end of transaction block
postgres=!# 
postgres=!# commit and chain; ## commit 这里相当于先回滚 && 继承
ROLLBACK
postgres=*# show timezone;
      TimeZone       
---------------------
 America/Los_Angeles
(1 row)

postgres=*# set timezone = 'PRC';
SET
postgres=*# show timezone;
 TimeZone 
----------
 PRC
(1 row)

postgres=*# commit and chain; ## commit 提交 && 继承
COMMIT
postgres=*# show timezone;
 TimeZone 
----------
 PRC
(1 row)

postgres=*# rollback and chain; ## 无东西可以回滚
ROLLBACK
postgres=*# show timezone;
 TimeZone 
----------
 PRC
(1 row)

postgres=*# reset timezone;
RESET
postgres=*# show timezone;
      TimeZone       
---------------------
 America/Los_Angeles
(1 row)

postgres=*# rollback and chain; ## rollback 这里相当于先回滚 && 继承
ROLLBACK
postgres=*# show timezone;
 TimeZone 
----------
 PRC
(1 row)

postgres=*#
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