Returning Stats from a Custom Function

[aemonm]

Is there a way to

1. run TiPG's filters on a table to get a subset of data
2. Perform some sort of calculation on the results - say, MAX(area), just COUNT(*) and return just a single row with those results?

I've attempted to do this with a custom function, but it looks like the filters are applied after function is run and the function must therefore return all rows. I just want a single tuple returned in this case, not the full set.