Hi,
Am 25.10.2012 um 20:22 schrieb Kevin Grittner:
>
> The idea is to model actual costs on your system. You don't show
> your configuration or describe your hardware, but you show an
> estimate of retrieving over 4000 rows through an index and describe a
> response time of 4 seconds, so you must have some significant part of
> the data cached.
Sure my effective_cache_size 10 GB
But my right Table has the size of 1.2 TB (yeah Terra) at the moment (partitioned a 40GB slices) and has 3 * 10^9
records
My left table has only the size of 227MB and 1million records. Peanuts.
> I would see how the workload behaves with the following settings:
>
> effective_cache_size = <your shared_buffers setting plus what the OS
> shows as cached pages>
> seq_page_cost = 1
> random_page_cost = 2
> cpu_tuple_cost = 0.05
>
> You can set these in a session and check the plan with EXPLAIN. Try
> various other important important queries with these settings and
> variations on them. Once you hit the right factors to model your
> actual costs, the optimizaer will make better choices without needing
> to tinker with it each time.
i've played with that already ….
NESTED LOOP -> GOOD
SEQSCAN -> VERY BAD
SET random_page_cost = 4;
2012-08-14' AND '2012-08-30' -> NESTED LOOP
2012-08-13' AND '2012-08-30' -> SEQSCAN
SET random_page_cost = 2;
2012-08-14' AND '2012-08-30' -> NESTED LOOP
2012-08-07' AND '2012-08-30' -> NESTED LOOP
2012-08-06' AND '2012-08-30' -> SEQSCAN
SET random_page_cost = 1;
2012-08-14' AND '2012-08-30' -> NESTED LOOP
2012-08-07' AND '2012-08-30' -> NESTED LOOP
2012-07-07' AND '2012-08-30' -> NESTED LOOP
2012-07-06' AND '2012-08-30' -> SEQSCAN
The thing is ..
- You can alter what you want. The planner will switch at a certain time range.
- There is not one case, where the SEQSCAN-Method will be better .. It's not possible.
So the only way to tell the planner that he's doomed is
SET enable_seqscan=0
which is not very elegant. (Query Hints would be BTW jehovah!)
You would be forced to write something like this:
var lastValueEnable_seqscan = "SHOw enable_seqscan"
SET enable_seqscan=0;
SELECT ...
SET enable_seqscan=lastValueEnable_seqscan;
Kind regards
Andy
--
Andreas Böckler
andy@boeckler.org