Re: Slow SELECT by primary key? Postgres 9.1.2

On Mon, May 27, 2013 at 9:02 AM, John Mudd <johnbmudd(at)gmail(dot)com> wrote:
> Postgres 9.1.2 on Ubuntu 12.04
>
> Any reason why a select by primary key would be slower than a select that
> includes an ORDER BY? I was really hoping using the primary key would give
> me a boost.
>
> I stopped the server and cleared the O/S cache using "sync; echo 3 >
> /proc/sys/vm/drop_caches" between the runs.
>
>
>
> test=# VACUUM ANALYZE test_select;
> VACUUM
>
> (stopped postgres; reset O/S cache; started postgres)
>
> test=# explain analyze SELECT * FROM test_select WHERE key1 >= 500000 ORDER
> BY key1, key2, key3, id LIMIT 1;
> QUERY PLAN
> --------------------------------------------------------------------------------------------------------------------------------------
> Limit (cost=0.00..0.08 rows=1 width=21) (actual time=12.599..12.600 rows=1
> loops=1)
> -> Index Scan using my_key on test_select (cost=0.00..41895.49
> rows=498724 width=21) (actual time=12.597..12.597 rows=1 loops=1)
> Index Cond: (key1 >= 500000)
> Total runtime: 12.678 ms
>
> (stopped postgres; reset O/S cache; started postgres)

why are you flushing postgres/os cache? when you do that, you are
measuring raw read time from disks. Typical disk seek time is
measured in milliseconds so the timings are completely appropriate
once you remove caching effects. Hard drives (at least, the spinning
kind) are slow and one of the major challenges of database and
hardware engineering is working around their limitations. Fortunately
it looks like faster storage will soon be commonplace for reasonable
prices.

merlin