For all intents and purposes, PostgreSQL has less of a need for CREATE PROCEDURE than other databases aside from looking more like other databases.
For example in SQL Server -> 2005 - although you can write functions that return tables and so forth, you have to resort to writing
CLR functions marked as unsafe to actually update data in a stored function. This gets pretty messy and has its own limitations so you have no choice but to use a stored procedures, which can not be called from within an SQL query.
In MySQL 5.1 the abilities of functions are even more limiting - they can't even return a dataset.
In PostgreSQL, you can
write a function marked as VOLATILE that updates data and that can do all sorts of wacky things that are useful
but considered by some to be perverse such as the following:
SELECT rule_id, rule_name, fnprocess_rule(rule_id) As process_result
WHERE brules.category = 'Pay Employees'
ORDER BY brules.rule_order
Another thing stored procedures can usually do that functions can not is to return multiple result sets. PostgreSQL can simulate such behavior
by creating a function that returns a set of refcursors. See this .NET example Getting full results in a DataSet object: Using refcursors way down the page,
that demonstrates creating a postgresql function that returns a set of refcursors to return multiple result sets using the Npgsql driver.
Prior to PostgreSQL 8.1, people could yell and scream, but PostgreSQL doesn't support Output Parameters. As weird as it is for a function to support
such a thing, PostgreSQL 8.1+ do support output parameters and ODBC drivers and such can even use the standard CALL interface to grab those values.
At a glance it appears that PostgreSQL functions do all that stored procedures do plus more. So the question is, is there any reason
for PostgreSQL to support bonafide stored procedures aside from the obvious To be more compatible with other databases and not have to answer the philosophical question, But you really don't support stored procedures?.
There must be some efficiency benefits to declaring something as a store procedure and having it called in that way. Not quite sure if anyone has done benchmarks on that.
So for the time being PostgreSQL functions have the uncanny role of having a beak like a duck and the flexibility of a beaver, but having the makeup of a Platypus.