SELECT 'NOW()' - OK, SELECT 'CLOCK_TIMESTAMP()' - ERROR

[Piotr Gasidło <quaker@xxxxxxxxxxxxxx>]

Hello,

I found strange PostgreSQL 9.3 behavior:

> select now()::timestamp, 'now()'::timestamp;

            now             |         timestamp          

----------------------------+----------------------------

 2014-08-22 08:34:00.883268 | 2014-08-22 08:34:00.883268

Second column is now() in single apostrophes.

Now, I tried similar function, clock_timestamp() and get:

> select clock_timestamp()::timestamp, 'clock_timestamp()'::timestamp;

ERROR:  invalid input syntax for type timestamp: "clock_timestamp()"

LINE 1: select clock_timestamp()::timestamp, 'clock_timestamp()'::ti...

                                               ^

Why is NOW() so special? Where is it documented? And why not working with other timestamp returning internal functions?

> select version();

                                                               version                                                                

--------------------------------------------------------------------------------------------------------------------------------------

 PostgreSQL 9.3.4 on amd64-portbld-freebsd10.0, compiled by FreeBSD clang version 3.3 (tags/RELEASE_33/final 183502) 20130610, 64-bit

(1 wiersz)

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Piotr Gasidło