string function - "format" function proposal

Hello

I am returning back to string functions. For me, the most important
function isn't commited still. There was discussion about "format" or
"sprintf" fuction. So I'll do a small resume.

goal: to get function that helps with formatting a message texts and
helps with building a SQL commands (used as dynamic SQL)

propsals:

* "format" function - uses same formatting as PL/pgSQL RAISE statement
* "sprintf" function

Itagaki objectives to "format" function:
* there are not possibility put two parameters without a space between
* it is too simple

My objectives to sprintf function:
* it is designed to different environment than SQL - missing support
NULL, missing support for date, timestamp, boolean, ...
* it is too complex, some parameters has different meaning for different tags
* we have a "to_char" function for complex formatting now.

Now I propose a compromise - "format" function with only three tags:

%s .. some string
%i  .. SQL identifier
%l  .. string literal

using a NULL:

for %s NULL is transformed to empty string - like "concat"
for %i NULL raises an exception
for %l NULL is transformed to ' NULL '  string.

This system is still simple and enough.

Implemented sprintf function can be moved to "sprintf" contrib module.

comments

Regards

Pavel Stehule