apt-get question

[Jorge Santos <jsf@dep3.fciencias.unam.mx>]

Hello, I want apt-get to get some packages from unstable (using
something like apt-get install postgresql/unstable), but for all the
other packages I want the versions in testing (so that doing an
'apt-get install postgresql install the version from testing), but I
think if I just edit sources.list and add the unstable sources then
the following 'apt-get install foo' will install the version from
unstable (when I whould like it to install the version from testing),
I hope I stated my problem clearly :)  Does anyone knows how I can
achieve this?