PostgreSQL timestamp parse in gram.y (‘ ‘ AT TIME ZONE ‘ ‘)
背景
去哪儿的AVEN兄弟提到的一个问题：
为什么当前时区是+8的情况，结果感觉上有点不对劲？
postgres=# show timezone;
TimeZone
----------
PRC
(1 row)
postgres=# select extract(epoch from 'today'::timestamptz);
date_part
------------
1430323200
(1 row)
postgres=# select extract(epoch from 'today' at time zone '0');
date_part
------------
1430323200
(1 row)
以上相等，不太对劲
postgres=# select extract(epoch from 'today' at time zone '8');
date_part
------------
1430294400
(1 row)
我们首先要看一下这个语法是怎么解析的。
src/backend/parser/gram.y
EXTRACT的写法会解析为调用函数date_part
| EXTRACT '(' extract_list ')'
{
$$ = (Node *) makeFuncCall(SystemFuncName("date_part"), $3, @1);
}
extract_list:
extract_arg FROM a_expr
{
$$ = list_make2(makeStringConst($1, @1), $3);
}
| /*EMPTY*/ { $$ = NIL; }
;
而 AT TIME ZONE 的写法实际调用了函数timezone
| a_expr AT TIME ZONE a_expr %prec AT
{
$$ = (Node *) makeFuncCall(SystemFuncName("timezone"),
list_make2($5, $1),
@2);
}
这两个函数如下：
postgres=# \df+ date_part
List of funct
ions
Schema | Name | Result data type | Argument data types | Type | Security | Volatility | Owner | Languag
e | Source code | Description
------------+-----------+------------------+-----------------------------------+--------+----------+------------+----------+--------
--+--------------------------------------------------------------------------+---------------------------------------------
pg_catalog | date_part | double precision | text, abstime | normal | invoker | stable | postgres | sql
| select pg_catalog.date_part($1, cast($2 as timestamp with time zone)) | extract field from abstime
pg_catalog | date_part | double precision | text, date | normal | invoker | immutable | postgres | sql
| select pg_catalog.date_part($1, cast($2 as timestamp without time zone)) | extract field from date
pg_catalog | date_part | double precision | text, interval | normal | invoker | immutable | postgres | interna
l | interval_part | extract field from interval
pg_catalog | date_part | double precision | text, reltime | normal | invoker | stable | postgres | sql
| select pg_catalog.date_part($1, cast($2 as pg_catalog.interval)) | extract field from reltime
pg_catalog | date_part | double precision | text, time with time zone | normal | invoker | immutable | postgres | interna
l | timetz_part | extract field from time with time zone
pg_catalog | date_part | double precision | text, time without time zone | normal | invoker | immutable | postgres | interna
l | time_part | extract field from time
pg_catalog | date_part | double precision | text, timestamp with time zone | normal | invoker | stable | postgres | interna
l | timestamptz_part | extract field from timestamp with time zone
pg_catalog | date_part | double precision | text, timestamp without time zone | normal | invoker | immutable | postgres | interna
l | timestamp_part | extract field from timestamp
(8 rows)
postgres=# \df+ timezone
List of functions
Schema | Name | Result data type | Argument data types | Type | Security | Volatility | Own
er | Language | Source code | Description
------------+----------+-----------------------------+---------------------------------------+--------+----------+------------+-----
-----+----------+-------------------+----------------------------------------
pg_catalog | timezone | time with time zone | interval, time with time zone | normal | invoker | immutable | post
gres | internal | timetz_izone | adjust time with time zone to new zone
pg_catalog | timezone | timestamp without time zone | interval, timestamp with time zone | normal | invoker | immutable | post
gres | internal | timestamptz_izone | adjust timestamp to new time zone
pg_catalog | timezone | timestamp with time zone | interval, timestamp without time zone | normal | invoker | immutable | post
gres | internal | timestamp_izone | adjust timestamp to new time zone
pg_catalog | timezone | time with time zone | text, time with time zone | normal | invoker | volatile | post
gres | internal | timetz_zone | adjust time with time zone to new zone
pg_catalog | timezone | timestamp without time zone | text, timestamp with time zone | normal | invoker | immutable | post
gres | internal | timestamptz_zone | adjust timestamp to new time zone
pg_catalog | timezone | timestamp with time zone | text, timestamp without time zone | normal | invoker | immutable | post
gres | internal | timestamp_zone | adjust timestamp to new time zone
(6 rows)
我们接下来分解一下以上查询：
postgres=# select
extract(epoch from 'today'::timestamptz),
date_part('epoch', 'today'::timestamptz),
'today'::timestamptz;
date_part | date_part | timestamptz
------------+------------+------------------------
1430323200 | 1430323200 | 2015-04-30 00:00:00+08
(1 row)
postgres=# select pg_typeof('today'::timestamptz);
pg_typeof
--------------------------
timestamp with time zone
(1 row)
这个查询用到了timestamptz_part
对于AT TIME ZONE的写法，因为对应的timezone函数可能输出带时区，或不带时区的值，所以我们需要区分一下，
但是对于date_part来说，结果是一样的，因为date_part也支持带时区和不带时区的参数。
postgres=# select
extract(epoch from 'today' at time zone '0'),
date_part('epoch', timezone('0', 'today'::timestamptz)),
timezone('0', 'today'::timestamptz),
timezone('0', 'today');
date_part | date_part | timezone | timezone
------------+------------+---------------------+---------------------
1430323200 | 1430323200 | 2015-04-29 16:00:00 | 2015-04-29 16:00:00
(1 row)
postgres=# select pg_typeof(timezone('0', 'today'));
pg_typeof
-----------------------------
timestamp without time zone
(1 row)
这个查询用到了timestamp_part，timestamptz_zone函数
结果一致的原因非常明显，因为我们用到的时间：2015-04-30 00:00:00+08 等于 2015-04-29 16:00:00。
调用的函数分别为：
timestamptz_part(text, timestamp with time zone)
timestamp_part(text, timestamp without time zone)
现在可以解释为什么extract(epoch from ‘today’ at time zone ‘8’)的结果有点问题了。
postgres=# select
date_part('epoch', timezone('8', 'today')),
timezone('8', 'today');
date_part | timezone
------------+---------------------
1430294400 | 2015-04-29 08:00:00
(1 row)
参考
1. timestamptz_part，timestamp_part，timestamptz_zone
src/backend/utils/adt/timestamp.c
2. src/backend/parser/gram.y