How to relocate temporary files

I have a database that's about 18GBs.
[base]# du -hs 17165
18G     17165

Occasionally someone runs a query and they get
"could not write block 460593 of temporary file: No space left on device"
At present the filesystem for the database has about 3GB free.

I'd like to give the temporary files a filesystem of their own.
What would be the preferred method?

Thanks
Richard Ray