Re: PostgreSQL and Linux 2.6 kernel.

Actually it hasn't been my experience either. Most of my queries against
the database, large and small are either a little quicker or no real
difference. I have only really noticed big differences under stress when
memory (RAM) is being squeezed. The main winner on 2.6 seems to be
write performance and memory management.

Unfortunately I only have one test machine and I can't really keep
switching between 2.4 and 2.6 to do the comparisons. I had written
down 27 timings from a set of SQL of varying complexity using the 2.4
kernel. Each SQL statement was executed 10 times and the average of
the last 5 was used. I can only really compare those timings against the
new installation on 2.6. I know that this is not ideal "real world" testing,
but it is good enough for me at the moment. Unless anyone has
contradictory indications then I will proceed with 2.6.

I did increase the default stats target from 10 to 50 and re-analysed.
The explain numbers are slightly different, but the time to run was
almost the same. Not surprising since the plan was the same.

QUERY PLAN
Merge Join (cost=0.00..192636.20 rows=2845920 width=92)
Merge Cond: ("outer".reqt_id = "inner".reqt_id)
-> Index Scan using order_reqt_pkey on order_reqt r (cost=0.00..52662.40
rows=2206291 width=6)
-> Index Scan using staff_book_idx2 on staff_booking (cost=0.00..102529.28
rows=2845920 width=90)

On 3 Apr 2004 at 10:59, Josh Berkus wrote:

Gary,

> There are no indexes on the columns involved in the update, they are
> not required for my usual select statements. This is an attempt to
> slightly denormalise the design to get the performance up comparable
> to SQL Server 2000. We hope to move some of our databases over to
> PostgreSQL later in the year and this is part of the ongoing testing.
> SQLServer's query optimiser is a bit smarter that PostgreSQL's (yet)
> so I am hand optimising some of the more frequently used
> SQL and/or tweaking the database design slightly.

Hmmm ... that hasn't been my general experience on complex queries. However,
it may be due to a difference in ANALYZE statistics. I'd love to see you
increase your default_stats_target, re-analyze, and see if PostgreSQL gets
"smarter".

--
-Josh Berkus
Aglio Database Solutions
San Francisco

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