I'd first make sure (I mean: absolutely sure) that the JDBC driver
doesn't try to hold the result set in memory.

How is memory usage/GC activity when "the connection freezes"?

May we see a relevant code snippet ?

Arne

I could imagine a hairy query that can't produce any results without
collecting and sorting a huge number of records first.  Perhaps the
query requires sequentially scanning the entire database.

I think what you want to do is find out what sort of strategy it is
using to execute the query.  Perhaps then you will be able to prod it
along with an index, an aux table etc.