Large difference between elapsed time and run time for queries

Hello,

While running benchmarks for my database, I am seeing a large difference in the elapsed time (from stats collected in the logs) and run time (running explain analyze on the query using ./psql <database>) for each of my queries.  The database is being ran on a sunfire 880 with 4 750mhz processors with 8 G RAM running solaris 8

I am simulating 200 user connections each running 6 select queries on 1 indexed table with 50,000 records. The elapsed time for the queries average around 2.5 seconds while if I run the query using explain analyze while the test is running, the run time is around 300 ms although it takes much longer (few seconds) to display the results.  If I reduce the number of concurrent connections to 100 then the run time and elapsed time for the queries are the same.

I have tried numerous configurations in the postgresql.conf file.  I have set the shared_buffers with values ranging from 75 MB to 4000MB with no luck.  I have also tried increasing the sort_mem with no luck.

When the test is running, the cpu is well over 50% idle and iostat shows that the processes are not waiting for i/o and disk usage percentage is low.

Any help would be appreciated.

Thanks.