getting execution plans with multiple database connections

Hi,

I am running OSDL-DBT3 test against PostgreSQL.  I found performance
difference between the runs even though the data and queries are the
same.  I tried to study this problem by getting execution plans at the
beginning of each test.  The following script gets execution plan for 22
queries, except query 15.

i=1
while [ $i -le 22 ]
do
        if [ $i -ne 15 ]; then
                $dbdriver_pgsql_path/explain_single_query.sh $i
$run_dir/plan/power_query$i.txt
        fi
        let "i=$i+1"
done

The script 'explain_single_query.sh' takes 2 parameters: query_number
and output file name.

It works for power test(single database connection).  In other words, I
can get execution plans for all the 21 queries if there is only one
stream connecting to the database.

But for the throughput test(multiple database connections), some query
execution plan files are blank.  It seems to be random, for example, for
the run at:
http://khack.osdl.org/stp/277495/results/plan/
throughput_stream1_query14.txt, throughput_stream3_query4.txt,
throughput_stream3_query5.txt, throughput_stream4_query4.txt,
throughput_stream4_query18.txt are of size 0.  And if I do the run
again, a different set of files are blank.

Can anyone give me an explanation?
Thanks,
--
Jenny Zhang
Open Source Development Lab Inc
12725 SW Millikan Way
Suite 400
Beaverton, OR 97005
(503)626-2455 ext 31