Re: Is `DATE` a function?

jinser <aimer(at)purejs(dot)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