One of the very handy features introduced in PostgreSQL 8.4 is the new aggregate function called array_agg which is a companion function to the unnest function we discussed earlier. This takes a set of elements similar to what COUNT, SUM etc do and builds an array out of them. This approach is faster than the old used array_append , array_accum since it does not rebuild the array on each iteration.

Mas:

http://www.postgresonline.com/journal/index.php?/archives/126-PostgreSQL-8.4-Faster-array-building-with-array_agg.html