PgQuery is a plugin for QGIS which allows you to execute sql with postgis functionality, and load the results directly as layers with geometry.
a longer writeup can be found here,
As a test, I executed the following query to get all the building footprints in a given neighborhood
(SELECT * FROM neighboorhoods WHERE neighboorhoods.pri_neigh = 'Lincoln Park') AS hood,
WHERE st_contains(hood.geom,"Buildings".the_geom) = true
Check it out! this should be really useful for you!!!