Re: A Better Way? (Multi-Left Join Lookup)

On 20 Jul 2012, at 22:30, David Johnston wrote:

> Hi!
>
> Can someone please point me to a resource (or suggest a solution) that will improve the performance of this query?  I
havesome thoughts but figure I should avoid reinventing the wheel since this seems like something that has to have been
solvedalready.  
>
> I am working on a query where I have a list of identifiers (sample set has about 8,500 records) and I have three
otherqueries that return a subset of these 8,500 identifiers 
>
> Basic query is designed as such:
>
> WITH
>   full_set AS ( ) -- 8,500 records
> , sub_1 AS () -- also about 8,500
> , sub_2 AS () -- maybe 5,000
> , sub_3 AS () - - maybe 3,000
> SELECT full_set.*
> , COALESCE(sub_1.field, FALSE)
> , COALESCE(sub_2.field, FALSE)
> , COALESCE(sub_2.field, FALSE)
>
> FROM full_set
> LEFT JOIN sub_1
> LEFT JOIN sub_2
> LEFT JOIN sub_3
>
> The goal is to output a boolean for each record in “full_set” specifying whether a corresponding records exists in
thesub-set.  If the record exists “sub_x.field” is defined to be TRUE and thus is output otherwise sub_x.field is NULL
andcoalesce returns FALSE. 

You are creating a product of the result sets for sub_1 to _3 there, while you only seem to need the union of the
three.

Perhaps something like this is what you're after?

WITH
  full_set AS ( )
, subs AS (
  SELECT 1 AS sub, TRUE AS field, ... FROM sub_1
  UNION ALL
  SELECT 2 AS sub, TRUE AS field, ... FROM sub_2
  UNION ALL
  SELECT 3 AS sub, TRUE AS field, ... FROM sub_3
)
SELECT ...
FROM full_set
LEFT JOIN subs

If you need those rows to be distinct, use UNION instead of UNION ALL, but the database needs to do more work for that.


Alban Hertroys

--
Screwing up is an excellent way to attach something to the ceiling.