SELECT performance issues on postgresql 9.1

Question

I am building a large postgres 9.1 database on ubuntu 12.04, with one table that holds about 80 million rows or so. Whenever I run a SELECT statement:

SELECT * FROM db WHERE ID=1;

It takes almost 2.5 minutes to execute the query which returns only a few thousand rows. After running a few diagnostics on the disk I/O, I think that is not the problem, but just in case below is the output from a diagnostic. (I have 2GB of RAM) I am not exactly sure what a good output is here, but it seems ballpark given stats found for other servers on the internet.

time sh -c "dd if=/dev/zero of=bigfile bs=8k count=500000 && sync"


500000+0 records in
500000+0 records out
4096000000 bytes (4.1 GB) copied, 106.969 s, 38.3 MB/s

real    1m49.091s
user    0m0.248s
sys     0m9.369s

I have modified postgresql.conf considerably, boosting the effective_cache to 75% of ram, shared_buffers to 25%, checkpoint_segments to 15, work_mem to 256MB, autovacuum, SHMMAX on the kernel, etc. I have had some performance increases, but not more than 5% better. Networking shouldnt be an issue since it still takes a long time even running on localhost. I am planning to add even more data, and the query time seems to be growing quickly with the number of rows.

It seems like I should be able to run these SELECT statements in a few seconds, not a few minutes. Any suggestions on where this bottleneck could be?

Answer 1

sorry if this is inexcusably obvious, but do you have an index on the ID column?

also, though I'm not blaming the disk, you merely tested sequential bandwidth, which tells you very little about latency. though I have to say that 38 MB/s is underwhelming even for that measure...