Re: Is `DATE` a function?

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 <tgl(at)sss(dot)pgh(dot)pa(dot)us> 于2023年10月7日周六 22:38写道：
>
> 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
>