Question

In the following query, the join to table title_deed is only required for the title_deed_no search criterion, and no fields are selected from it. If the @titleDeedNumber parameter is null, will a join still be performed? How much difference would this make to performance anyway? I am trying to replace dynamic generation of a big ugly T-SQL string, and the code that does that only adds a join if a title_deed_no value is supplied.

DECLARE @registrarId nchar(1)
DECLARE @titleDeedNumber nvarchar(50)

SELECT TOP 100
        vw.*
FROM    ACC.dbo.vw_Property_Nad vw
        INNER JOIN title_deed td ON ( @titleDeedNumber IS NULL )
                                    OR ( td.Prop_ID = vw.prop_id )
WHERE   vw.Prop_ID IS NOT NULL
        AND Registrar = isnull(@registrarId, vw.Registrar)
        AND td.title_deed_no = isnull(@titleDeedNumber, td.title_deed_no)

I have tried examning the execution plan, but it is just too busy with joins from the vw_Property_Nad view, and I’d rather get an expert opinion anyway.

Answer 1

In this case, probably not.

You’re better off with:

• an EXISTS not JOIN (ie semi-join not equi-join)
• (vw.Registrar = @registrarId OR @registrarId IS NULL)
• ditto for td.title_deed_no in the EXISTS

Also

• TOP without ORDER BY is useless
• Your JOIN condition implies CROSS JOIN -> you’d need DISTINCT to get it working
• vw implies view -> the query is more complex than what you expect. A view is a macro that expands

I’d also consider an IF or UNION ALL

SELECT
        vw.*
FROM    ACC.dbo.vw_Property_Nad vw
WHERE   vw.Prop_ID IS NOT NULL
        AND (vw.Registrar = @registrarId OR @registrarId IS NULL)
        AND @titleDeedNumber IS NULL
UNION ALL
SELECT
        vw.*
FROM    ACC.dbo.vw_Property_Nad vw
WHERE   vw.Prop_ID IS NOT NULL
        AND (vw.Registrar = @registrarId OR @registrarId IS NULL)
        AND @titleDeedNumber IS NOT NULL
        AND EXISTS (...)
ORDER BY something