crazy logging from PG 8.4

After several days of running a simple query over a very large dataset
(more than a terabyte), I logged in to note that postgresql had
decided to log *billions* of log lines.

The query looks like this:

create table result_table as
select TEXTFIELD, count(distinct INTFIELD) from SOME_HUGE_TABLE group
by TEXTFIELD;

At some point, PG started logging stuff that looked like this (over
and over again):

SELECT 2704204951 user@dbname LOG:  00000: performsort starting: CPU 0.00....

The 2704204951 number corresponds to the "%l" (session line number)
in log_line_prefix.  Thus, by the time I saw this, it had logged 2.7
billion lines.


An strace of the process showed this, over and over again:

getrusage(....)
write(2, <logging info>)

and nothing else.  What happened here?  It seems like PG just went
crazy and got itself into some sort of loop. I had to kill the query.

This is PostgreSQL 8.4.13 on x86_64, Linux, using Scientific Linux 6.4.

--
Jon