unix_timestamp函数指定不同时区——返回值跟mysql不相符

【标题描述】:
【测试类型：SQL功能/存储功能/接口功能/工具功能/性能/并发/压力长稳/故障注入/安全/资料/编码规范】【测试版本：x.x.x】问题描述

【操作系统和硬件信息】(查询命令: cat /etc/system-release, uname -a):

【测试环境】(单机/1主x备x级联备):

【被测功能】:unix_timestamp函数指定不同时区

【测试类型】:

【数据库版本】(查询命令: gaussdb -V): 输入图片说明

【预置条件】:
mysql验证默认信息为：
输入图片说明

【操作步骤】(请填写详细的操作步骤):
set timezone=default;
select unix_timestamp('1970-01-01+00');
select unix_timestamp('1970-01-01+08');
select unix_timestamp('1970-01-01 00:00:00');
select unix_timestamp('1970-01-01 08:00:00');
select unix_timestamp('1970-01-01 00:00:00+00:00');
select unix_timestamp('1970-01-01 08:00:00+00:00');
select unix_timestamp('1970-01-01 00:00:00+08:00');
select unix_timestamp('1970-01-01 08:00:00+08:00');

set timezone='+00';
select unix_timestamp('1970-01-01+00');
select unix_timestamp('1970-01-01+08');
select unix_timestamp('1970-01-01 00:00:00');
select unix_timestamp('1970-01-01 08:00:00');
select unix_timestamp('1970-01-01 00:00:00+00:00');
select unix_timestamp('1970-01-01 08:00:00+00:00');
select unix_timestamp('1970-01-01 00:00:00+08:00');
select unix_timestamp('1970-01-01 08:00:00+08:00');

set timezone='+04';
select unix_timestamp('1970-01-01+00');
select unix_timestamp('1970-01-01+08');
select unix_timestamp('1970-01-01 00:00:00');
select unix_timestamp('1970-01-01 08:00:00');
select unix_timestamp('1970-01-01 00:00:00+00:00');
select unix_timestamp('1970-01-01 08:00:00+00:00');
select unix_timestamp('1970-01-01 00:00:00+08:00');
select unix_timestamp('1970-01-01 08:00:00+08:00');

set timezone='+08';
select unix_timestamp('1970-01-01+00');
select unix_timestamp('1970-01-01+08');
select unix_timestamp('1970-01-01 00:00:00');
select unix_timestamp('1970-01-01 08:00:00');
select unix_timestamp('1970-01-01 00:00:00+00:00');
select unix_timestamp('1970-01-01 08:00:00+00:00');
select unix_timestamp('1970-01-01 00:00:00+08:00');
select unix_timestamp('1970-01-01 08:00:00+08:00');

set timezone='-08';
select unix_timestamp('1970-01-01+00');
select unix_timestamp('1970-01-01+08');
select unix_timestamp('1970-01-01 00:00:00');
select unix_timestamp('1970-01-01 08:00:00');
select unix_timestamp('1970-01-01 00:00:00+00:00');
select unix_timestamp('1970-01-01 08:00:00+00:00');
select unix_timestamp('1970-01-01 00:00:00+08:00');
select unix_timestamp('1970-01-01 08:00:00+08:00');

【预期输出】:
default——0,0,0,0,0,0,0,0,0
输入图片说明
+00:00——0,28800,0,28800，0,28800,0,28800

-08:00——28800,57600,28800,57600，28800,57600，28800,57600

+08:00——0,0,0,0，0,0,0,0

+04:00——0,14400,0,14400，0,14400,0,14400

【实际输出】:
default——14400,43200,14400,43200,14400,43200,14400，43200
输入图片说明
+00:00——0,28800,0,28800，0,28800,0,28800

-08:00——0,0,0,0，0,0，0,0

+08:00——28800,57600,28800,57600，28800,57600,28800,57600

+04:00——14400,43200,14400,43200，14400,43200,14400,43200

【原因分析】:

这个问题的根因
问题推断过程
还有哪些原因可能造成类似现象
该问题是否有临时规避措施
问题解决方案
预计修复问题时间

【日志信息】(请附上日志文件、截图、coredump信息):

【测试代码】:

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已回归
openGauss 5.1.0 build b5a8d5b0
set timezone=default;
输入图片说明

set timezone='+00';
输入图片说明

set timezone='+04';
输入图片说明

set timezone='-08';
输入图片说明

set timezone='+08';

selectdb=# select unix_timestamp('1970-01-01+00');
select unix_timestamp('1970-01-01 00:00:00');
select unix_timestamp('1970-01-01 08:00:00');
select unix_timestamp('1970-01-01 00:00:00+00:00');
select unix_timestamp('1970-01-01 08:00:00+00:00');
select unix_timestamp('1970-01-01 00:00:00+08:00');
select unix_timestamp('1970-01-01 08:00:00+08:00'); unix_timestamp
----------------
              0
(1 row)

db=# select unix_timestamp('1970-01-01+08');
 unix_timestamp
----------------
              0
(1 row)

db=# select unix_timestamp('1970-01-01 00:00:00');
 unix_timestamp
----------------
              0
(1 row)

db=# select unix_timestamp('1970-01-01 08:00:00');
 unix_timestamp
----------------
              0
(1 row)

db=# select unix_timestamp('1970-01-01 00:00:00+00:00');
 unix_timestamp
----------------
              0
(1 row)

db=# select unix_timestamp('1970-01-01 08:00:00+00:00');
 unix_timestamp
----------------
              0
(1 row)

db=# select unix_timestamp('1970-01-01 00:00:00+08:00');
 unix_timestamp
----------------
              0
(1 row)

db=# select unix_timestamp('1970-01-01 08:00:00+08:00');
 unix_timestamp
----------------
              0
(1 row)

gsql ((openGauss 5.0.1 build f766addf) compiled at 2023-10-07 18:07:51 commit 0 last mr )
1.set timezone=default;
输入图片说明

2.set timezone='+00';

3.set timezone='+04';

4.set timezone='+08';

5.set timezone='-08';