Re: Very slow queries on 8.1

Michael Fuhr wrote:

>On Thu, Nov 17, 2005 at 10:38:50AM -0500, David Rysdam wrote:
>
>
>>I'm porting an application from Sybase and I've noticed that similar
>>application functions take 2 to 3 times longer on postgres than they
>>used to on the same machine running under Sybase.  I've tried changing
>>various "performance tuning" parameters, such as shared_buffers,
>>effective_cache_size, etc but there's little or no effect.
>>
>>
>
>What's your OS and hardware environment and what are your non-default
>PostgreSQL settings?
>
>
>
Fedora Core 2, dual 2.8 GHz, 2 GB ram.

shared_buffers = 10000
effective_cache_size = 100000

>>Right now, I'm working on a test case that involves a table with ~360k
>>rows called "nb.sigs".  My sample query is:
>>
>>select * from nb.sigs where signum > 250000
>>
>>With no index, explain says this query costs 11341.  After CREATE INDEX
>>on the signum field, along with an ANALYZE for nb.sigs, the query costs
>>3456 and takes around 4 seconds to return the first row.
>>
>>
>
>Please post the EXPLAIN ANALYZE output for the query -- that'll
>show us the query plan, the accuracy of the planner's row count
>estimate, and how long the query takes to execute on the server.
>It might also be useful to see the table definition and the output
>of the following query:
>
>SELECT null_frac, n_distinct, correlation
>FROM pg_stats
>WHERE schemaname = 'nb' AND tablename = 'sigs' AND attname = 'signum';
>
>
>
The first things would be problematic to supply, since they are actually
on a computer that doesn't have access to the Internet or to the machine
I'm writing this on.  As for the query:

Row  null_frac  n_distinct  correlation
1        0               -1               1

>What client interface are you using?  If the query returns a lot
>of rows then you might benefit from using a cursor to fetch the
>result set in chunks; otherwise the client library is probably
>waiting for the entire result set to arrive before returning any
>rows to you.
>
That does seem to be the problem.  I've never worked with cursors, so
I'll have to see if I can fit our DB module into that mold.