Re: List Concatination

Richard,

I wanted to thank you for the concatination suggestion ... on testing,
a custom aggregate *was* faster than procedural concatination ... much
faster.

> But - if you don't care about the order of contacts you can define an
> aggregate function:
>
> create aggregate catenate(sfunc1=textcat, basetype=text, stype1=text, initcond1='');
>
> Then group by client and catenate(firstname || ' ' || lastname)
>
> You'll want to read the CREATE AGGREGATE page in the reference manual,
> replace textcat with your own routine that adds a comma and you'll need
> a finalisation routine to strip the final trailing comma.

Actually, if you use a sub-select as your data source, you can control
both the appearance and the order of the catenated values:

SELECT client, catenate(con_name)
FROM (SELECT client, (firstname || ' ' || lastname || ', '
FROM contacts ORDER BY lastname ASC) AS con_list
GROUP BY client;

This seems to work pretty well.

> Note that this is probably not a good idea - the ordering of the
> contacts will not be well-defined. When I asked about this Tom Lane was
> quite surprised that it worked, so no guarantees about long-term suitability.

Hmmm ... this feature is very, very, useful now that I know how to use
it. I'd love to see it hang around for future versions of PgSQL. Tom?

-Josh Berkus

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