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From: jinser <[email protected]>
To: Tom Lane <[email protected]>
Cc: jinser <[email protected]>
Cc: [email protected]
Subject: Re: Is `DATE` a function?
Date: Sun, 8 Oct 2023 00:18:55 +0800
Message-ID: <CAK3STzEs8ABpm6bhhHjNWmuskTFed2_+Fohk+JeBaA+o=X5s8g@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <CAK3STzGm-8_5yvyWm_SJALo1Wb1i3cMJrBj8=WPmVyxHdhmYrg@mail.gmail.com>
	<[email protected]>

Thank you so much for your explanation.
With the reminder of ”cast function“, I found a more detailed
explanation in the document that I missed before:
https://www.postgresql.org/docs/16/sql-expressions.html#SQL-SYNTAX-TYPE
-CASTS.

Thanks again everyone :)

Tom Lane <[email protected]> 于2023年10月7日周六 22:38写道：
>
> jinser <[email protected]> writes:
> > playground=# SELECT DATE('2022-01-13');
> >     date
> > ------------
> >  2022-01-13
> > (1 row)
>
> Sure, there are functions named date():
>
> postgres=# \df date
>                              List of functions
>    Schema   | Name | Result data type |     Argument data types     | Type
> ------------+------+------------------+-----------------------------+------
>  pg_catalog | date | date             | timestamp with time zone    | func
>  pg_catalog | date | date             | timestamp without time zone | func
> (2 rows)
>
> The reason these aren't explicitly documented is that they are intended as
> implementation support for casts.
>
> postgres=# \dC date
>                                       List of casts
>          Source type         |         Target type         |  Function   |   Implicit?
> -----------------------------+-----------------------------+-------------+---------------
>  date                        | timestamp with time zone    | timestamptz | yes
>  date                        | timestamp without time zone | timestamp   | yes
>  timestamp with time zone    | date                        | date        | in assignment
>  timestamp without time zone | date                        | date        | in assignment
> (4 rows)
>
> Hence, the preferred spelling is more like
>
>         select now()::date;
>
> or if you want to be SQL-spec-compatible,
>
>         select cast(now() as date);
>
> but for historical reasons we like to let you also write
>
>         select date(now());
>
> which is managed (in most cases) by naming cast implementation
> functions the same as the target type.
>
> > Another reason I think this is a function is that other types don't
> > seem to have the same behavior:
>
> > playground=# SELECT integer('123');
> > ERROR:  syntax error at or near "("
>
> You're running into a couple of things there: INTEGER is a reserved
> word, and the cast functions for that type are named after the
> internal type name "int4".
>
> postgres=# \dC integer
>                               List of casts
>    Source type    |   Target type    |      Function      |   Implicit?
> ------------------+------------------+--------------------+---------------
>  "char"           | integer          | int4               | no
>  bigint           | integer          | int4               | in assignment
>  bit              | integer          | int4               | no
>  boolean          | integer          | int4               | no
>  double precision | integer          | int4               | in assignment
>  integer          | "char"           | char               | no
>  ...
>
> postgres=# select int4('123');
>  int4
> ------
>   123
> (1 row)
>
> Note that none of these have anything to do with the syntax for
> a typed literal, which is "type-name quoted-literal" with no
> parentheses:
>
> postgres=# select date 'today';
>     date
> ------------
>  2023-10-07
> (1 row)
>
> postgres=# select integer '42';
>  int4
> ------
>    42
> (1 row)
>
> Some aspects of the behavior might look the same, but there
> are a lot of edge cases.
>
>                         regards, tom lane
>

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