The descriptions given above for how swap works actually only applies to Linux (real memory fills then it is swapped out to make room for more). Other Unix OS, notable Solaris and *BSD tend to be a little more intelligent about swap and frequently swap out unused stuff regardless of how much free RAM there is.
However, to get to the point of this post. On the Solaris Box - It is _likely_ that your /tmp directory is a "tmpfs memory filesystem" / aka Swap space.
MySQL defaults to /tmp for the temp tables it creates when doing _all_ queries. Thus queries are always going to be using tmpfs space - this is good since it means (usually) that your temp tables can be created and deleted in memory.
The amount of "swap space" available/used will also depend of the size of the /tmp directory.
Now Solaris defines Swap as the sum of physical memory not used + physical swap space. It is not just the disk based "space". Try the commands:
swap -l
df -k /tmp ; swap -s
To see just what your machine thinks is going on.
Hope this helps,
Paul.
