More aggregate functions?

[David Garamond <lists@zara.6.isreserved.com>]

What do people think of adding some more aggregate functions. These are 
the ones that MySQL has and PG doesn't:

- STD/STDDEV
- VARIANCE
- BIT_OR
- BIT_AND
- GROUP_CONCAT (for strings, added in MySQL 4.x)

Particularly, I find GROUP_CONCAT practical to quickly display 1-many 
relationship, e.g.:

 CREATE TABLE mommy (
   id INT PRIMARY KEY,
   name TEXT);
 CREATE TABLE child (
   id INT PRIMARY KEY,
   mommy_id INT REFERENCES(mommy(id)),
   name TEXT
 );

 SELECT
   mommy.name as mommy,
   GROUP_CONCAT(child.name SEPARATOR ", ") as children
 FROM mommy, child
 GROUP BY mommy.id;

would result in:

 mommy         children
 -----         -----------------
 dot           ellen, catherine
 cate          bridget, kerry, rory
 rachel        emma

Btw, I have written 1 or 2 of the above myself with CREATE AGGREGATE. 
But perhaps it would be nice if it's available as a builtin function or 
at least present in contrib/.

--
dave


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