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Re: Aggregate "rollup"

From: Joe Conway <mail(at)joeconway(dot)com>
To: mlw <pgsql(at)mohawksoft(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Aggregate "rollup"
Date: 2003-03-05 21:10:46
Message-ID: 3E6667D6.4020003@joeconway.com (view raw or flat)
Thread:
Lists: pgsql-hackers
mlw wrote:
> I had written a piece of code about two years ago that used the 
> aggregate feature of PostgreSQL to create an array of integers from an 
> aggregate, as:
> 
> select int_array_aggregate( column ) from table group by column
> 
> While it seems pointless to create an array on a select, it has a 
> purpose in OLAP. For instance, suppose you do this:
> 
> create table fast_lookup as select reference, 
> int_array_aggregate(result) from table group by result
> 
> The "fast_lookup" table now has all the result entries as an array in a 
> single row. In the systems that I have used this, it has provided a 
> dramatic improvement, especially when you have a high number of 
> identical "reference" entries in a classic "one to many" table.
> 
> The question is, would a more comprehensive solution be wanted? 
> Possible? Something like:
> 
> create table fast_lookup as select reference, aggregate_array( field ) 
> from table group by field
> 
> Where the function aggregate_array takes any number of data types.
> 
> Any thoughts? I think I need to fix the code in the current 
> /contrib/intagg anyway, so is it worth doing the extra work to included 
> multiple data types?

It's also useful in conjunction with statistically processing. There is 
a array_accum function in PL/R; I just made a post to the SQL list the 
other day on this.
(http://archives.postgresql.org/pgsql-sql/2003-03/msg00124.php)
Here's the meat of it:

CREATE OR REPLACE FUNCTION array_accum (_name, name)
RETURNS name[]
AS '$libdir/plr','array_accum'
LANGUAGE 'C';

CREATE AGGREGATE accumulate (
   sfunc = array_accum,
   basetype = name,
   stype = _name
);

regression=# SELECT accumulate(tablename) as cruft FROM pg_tables WHERE 
tablename LIKE 'c%';
cruft
---------------------------------------
{connectby_int,connectby_text,ct,cth}
(1 row)

See:
   http://www.joeconway.com/plr/doc/plr-aggregate-funcs.html
and download at:
   http://www.joeconway.com/plr/

I'd be happy to split the array functions out of PL/R and sumbit them to 
PATCHES if there is any interest.

Joe


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