Re: Review: listagg aggregate

On Fri, 2010-01-22 at 11:14 -0800, David E.Wheeler wrote:
> No performance issues

ISTM that this class of function is inherently dangerous performance
wise. 

* It looks incredibly easy to construct enormous lists. We should test
the explosion limit of this to see how it is handled. Perhaps we need
some parameter limits to control that, depending upon results.

* Optimizer doesn't consider whether the result type of an aggregate get
bigger as the aggregate processes more rows. If we're adding this
function we should give some thought in that area also, or at least a
comment to note that it can and will cause the optimizer problems in
complex queries.

-- Simon Riggs           www.2ndQuadrant.com