a spatial table's bounding box

Hello, 

Me question is a basic one for postGIS, I believe. It should have a primitive answer somewhere, but I don't have it. 

I want to get the extensions of  spatial table. Say that there is a spatial table named 'roads', and a some road geometry inside. I want to fetch the bounding box (envelope or extension) for that table, that means for all the roads that it has. 

According the OGC specification, this should be defined in the "geometry_columns" table. As four columns, "minX", "minY", "maxX" and "maxY". But I couldn't find it anywhere on my postgis. Yet.

From which table/view? With what SQL?

Thanks for your answers,

Aslı.