what does this mean - pg_psort.1922.0 in database directory?

Hi all!

I found strange files in my database directory.

ls /usr/local/pgsql/base/isrts/pg_psort.1922.* -l
-rw-------   1 postgres postgres 564108288 Oct  1 17:37 /usr/local/pgsql/base/isrts/pg_psort.1922.0
-rw-------   1 postgres postgres        0 Oct  1 16:57 /usr/local/pgsql/base/isrts/pg_psort.1922.1
-rw-------   1 postgres postgres        0 Oct  1 16:57 /usr/local/pgsql/base/isrts/pg_psort.1922.2
-rw-------   1 postgres postgres        0 Oct  1 16:57 /usr/local/pgsql/base/isrts/pg_psort.1922.3
-rw-------   1 postgres postgres        0 Oct  1 16:57 /usr/local/pgsql/base/isrts/pg_psort.1922.4
-rw-------   1 postgres postgres        0 Oct  1 16:57 /usr/local/pgsql/base/isrts/pg_psort.1922.5
-rw-------   1 postgres postgres        0 Oct  1 16:57 /usr/local/pgsql/base/isrts/pg_psort.1922.6

What does it mean?
Any ideas?

                Best regards, Natalia Makushina.