Re: 7.4 vs 7.3 ( hash join issue )

Dennis Bjorklund wrote:
> On Wed, 22 Sep 2004, Gaetano Mendola wrote:
>
>
>> Limit (cost=10.21..10.21 rows=1 width=24) (actual time=0.885..0.885 rows=0 loops=1)
>> Limit (cost=10.28..10.28 rows=1 width=24) (actual time=0.429..0.429 rows=0 loops=1)
>
>
> These estimated costs are almost the same, but the runtime differs a bit.
> This means that maybe you need to alter settings like random_page_cost,
> effective_cache and maybe some others to make the cost reflect the runtime
> better.
>
> Since the costs are so close to each other very small changes can make it
> choose the other plan. It's also very hard to make an estimate that is
> correct in all situations. That's why it's called an estimate after all.

Is not feseable.

That values are obtained with random_page_cost = 2, effective_cache_size = 20000,
cpu_tuple_cost = 0.01
increasing or decreasing random_page_cost this means increase or decrease both
costs:

random_page_cost = 1.5
hashjoin on => 8.47
hashjoin off => 8.53

random_page_cost = 3
hashjoin on => 13.70
hashjoin off => 13.76

so is choosen the hasjoin method in both cases.

In the other side the effective_cache_size doesn't affect this costs.

Decreasing the cpu_tuple_cost have the same effect

cpu_tuple_cost = 0.005
hashjoin on => 10.11
hashjoin off => 10.17

cpu_tuple_cost = 0.001
hashjoin on => 10.03
hashjoin off => 10.03

cpu_tuple_cost = 0.0005
hashjoin on => 10.01
hashjoin off => 10.01

And when the two costs are the same the hashjoin path is choosen.

I think cpu_tuple_cost less then 0.001 is not a good idea

I think the only way is set the hashjoin = off. Any other suggestion ?

Regards
Gaetano Mendola