format() with embedded to_char() formatter

format() function is very useful to construct formatted text,
but it doesn't support embedded formatter unlike sprintf() in C.
Of course, we can use to_char() functions for each argument value,
but embedded formatter would be more readable.

I'd like to propose %{...}s syntax, where format('%{xxx}s', arg)
is equivalent to format('%s', to_char(arg, 'xxx')). I think the
approach is better than implement C-like formatter because we
can reuse existing to_char() functions for the purpose.

Here are examples for the usage:

=# SELECT format('%{FM0000}s : %{YYYY-MM-DD}L', 123, current_timestamp);
format
---------------------
0123 : '2010-11-22'

=# SELECT format('CREATE TABLE partition_%{YYYYMMDD}s () INHERITS
parent', current_date);
format
----------------------------------------------------
CREATE TABLE partition_20101122 () INHERITS parent

Is it interesting? Comments welcome.

--
Itagaki Takahiro