Index build temp files

Greetings,

We were surprised recently to note that the temp files that are
created during a CREATE INDEX don't go into either a temp tablespace
set for the user or into the tablespace which the CREATE INDEX
specifies. Instead, they go only into base/pgsql_tmp/. This doesn't
allow for any flexibility in defining where to create these
potentially quite large sets of files.

Shouldn't these temp files be going into the temp tablespace for the
user creating the index instead..? Or perhaps into the tablespace
which the index is being created in?

Thanks,

Stephen