i want to optimize this query - sql

Question

i have the following tables :
dbo.Details
Name Type SubType SerialNumber
D_01 TxA STxA1 4
D_02 TxB STxB2 3
D_03 TxC STxC1 2
D_04 TxD STxD1 7
D_05 TxD STxD1 1
D_06 TxD STxD1 9
dbo.DetailsType
Code Name
TxA A
TxB B
TxC C
...
dbo.DetailsSubType
Code Type Name CustomOR
STxA1 TxA A1 1
STxA2 TxA A2 0
STxB1 TxB B1 1
STxB2 TxB B2 0
STxC1 TxC C1 1
STxC2 TxC C2 0
STxD TxD D1 1
I want to know what query (A or B) is optimal in your opinion, with explanation please:
QUERY A
CREATE PROCEDURE XXX
(
#type nvarchar(10),
#subType nvarchar(10) = null
)
AS
BEGIN
declare #custom bit = 0;
if (#subType is not null)
begin
select #custom = CustomOR from dbo.DetailsSubType where SubType = #subType
end
select
DTST.SubType,
DT.SerialNumber
from dbo.Details as DT
left join DetailsSubType as DTST
on DT.SubType = DTST.Code
where
DT.Type = #type
and
(
#subType is null or
(#custom = 0 and DTST.CustomOR= 0) or
(#custom = 1 and DT.SubType = #subType)
)
END
QUERY B
declare #custom bit = 0;
if (#subType is not null)
begin
select #custom = CustomOR from dbo.DetailsSubType where SubType = #subType
end
if (#custom = 0)
begin
select
DTST.SubType,
DT.SerialNumber
from dbo.Details as DT
left join DetailsSubType as DTST
on DT.SubType = DTST.Code
where
DT.Type = #type
and
DTST.CustomOR = 0
end
else
begin
select
DTST.SubType,
DT.SerialNumber
from dbo.Details as DT
left join DetailsSubType as DTST
on DT.SubType = DTST.Code
where
DT.Type = #type
and
(DTST.CustomOR = 1 and DT.SubType = #subType)
end

Answer 1

Unfortunately, neither may be optimal. I am guessing that your concern is related to performance and the execution plan of the query. The second method definitely gives SQL Server better opportunities for the optimization plan -- simply because OR is really hard to optimize.
But this doesn't take into account "parameter sniffing". There are lots of articles about this subject (here is a reasonable one).
Parameter sniffing means that SQL Server compiles the query the first time the stored procedure is called. This saves the overhead of recompiling the queries -- a savings that is important if you have lots of "small" queries. But a fool's bargain for larger queries -- because it does not take statistics on the table into account.
I would suggest that you look into articles about this. You may find that the second solution is sufficient. You may find that simply adding option recompile is sufficient. You may find that you want to construct the query as dynamic SQL -- hey, you know it will be recompiled anyway. But you'll be able to make a more informed decision.

Answer 2

You could consider writing three queries that partition your results, where each query handles exactly one of your OR predicates, and UNION ALL the results.
In pseudo code:
SELECT ... FROM ... WHERE #subType is null
UNION ALL
SELECT ... FROM ... WHERE #subType is NOT null AND DTST.CustomOR = 0 AND #custom = 0
UNION ALL
SELECT ... FROM ... WHERE #subType is NOT null AND DT.SubType = #subType AND #custom = 1
Having said that, what I actually think is that you should change your data model. It is extremely had (and very slow) to move forward with this setup. You probably haven't normalized your database properly.