Re: Is `DATE` a function?

jinser <aimer@purejs.icu> 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