Reducing information

[Jos é Vte Higón]

Hi list,
I have running mapserver with postgis and I want to serve a huge layer which
original datasource is a shapefile with 1Gb size. If I try to get the layer
with a full extent the request take long time to be visualized. In order to
solve my problem, I have configured two layers, the first one can be
visualized when the scale it's very small and the datasource is the original
shapefile into postgis, the other one only can be visualized when the scale
is bigger but I need a datasource smaller than the original. Finally I have
a unique layer accesing both with a wms connection type (as wms client).

How can I get a datasource smaller than the original shapefile (or postgis
table)?
Is my solution the correct approach to the problem?

Thanks in advance.