Re: "union" vs. left join

On Mar 3, 2009, at 3:26 PM, Emi Lu wrote:

> Good morning,
>
> Would it be possible to know that whether "union" is quicker than
> "left join" in a general sense?
>
> Queries:
> ========================
> (1) union
> select a.*, b.value
> from a
> left join b
> union
> select a.*, c.value
> from a
> left join c
>
>
> (2) left join
> select distinct a.*,
> case when ... then
> b.value
> else
> c.value
> from a
> left join b
> left join c
>
>
> Will (1) be more efficient than (2) or vice versa? Or it really
> depends on data
>
>
> Thanks a lot!
> Lu Ying
>

I would say try it in your situation...
I never did any real worl testing, but usually I grab a left join
before anything else.
I would suspect that the planner knows a bit better how to optimize
the left join version.

Ries