SQL Server conditional join techniques - sql-server

Question

I have come across this situation multiple times wherein I need to grab data from one or another table based on some parameter to the stored procedure. Let me clarify with an example. Suppose we need to grab some data from either an archived table or an online table and a bunch of other tables. I can think of 3 ways to accomplish this:
Use an if condition and store result in a temp table and then join temp table to other tables
Use an if condition and grab data either from archive table or online table and join other tables. The entire query will be duplicated except for the part of archive table or online table.
Use a union subquery
Query for Approach 1
create table #archivedOrOnline (Id int);
declare #archivedData as bit = 1;
if (#archivedData = 1)
begin
insert into #archivedOrOnline
select
at.Id
from
dbo.ArchivedTable at
end
else
begin
insert into #archivedOrOnline
select
ot.Id
from
dbo.OnlineTable ot
end
select
*
from
#archivedOrOnline ao
inner join dbo.AnotherTable at on ao.Id = at.Id;
-- Lots more joins and subqueries irrespective of #archivedData
Query for Approach 2
declare #archivedData as bit = 1;
if (#archivedData = 1)
begin
select
*
from
dbo.ArchivedTable at
inner join dbo.AnotherTable another on at.Id = another.Id
-- Lots more joins and subqueries irrespective of #archivedData
end
else
begin
select
*
from
dbo.OnlineTable ot
inner join dbo.AnotherTable at on ot.Id = at.Id
-- Lots more joins and subqueries irrespective of #archivedData
end
Query for Approach 3
declare #archivedData as bit = 1;
select
*
from
(
select
m.Id
from
dbo.OnlineTable ot
where
#archivedData = 0
union
select
m.Id
from
dbo.ArchivedTable at
where
#archivedData = 1
) archiveOrOnline
inner join dbo.AnotherTable at on at.Id = archiveOrOnline.Id;
-- Lots more joins and subqueries irrespective of #archivedData
Basically I am asking which approach to choose or if there is a better approach. Approach 2 will have a lot of duplicate code. The other 2 approaches remove code duplication. I even have the query plans but my knowledge of making decisions based on the query plan is limited. I always go with the approach which removes code duplication. If there is a performance issue, I may choose another approach.

Answer 1

Your approach 3 can work fine. You should definitely use UNION ALL not UNION though so SQL Server does not add operations to remove duplicates from the tables.
For best chances of success with approach 3 you would need to add an OPTION (RECOMPILE) hint so that SQL Server simplifies out the unneeded table reference at compile time at the expense of recompiling it on each execution.
If the query is executed too frequently to make that approach attractive then you may get an OK plan without it and filters with startup predicates to only access the relevant table at run time - but you may have problems with cardinality estimates with this more generic approach and it might limit the optimisations available and give you a worse plan than option 2.

Answer 2

If you don't mind extra unused columns in your results, you can represent such "IF"s with additional join conditions.
SELECT stuff
FROM MainTable AS m
LEFT JOIN ArchiveTable AS a ON #archivedData = 1 AND m.id = a.id
LEFT JOIN OnlineTable AS o ON #archivedData <> 1 AND m.id = o.id
;
If the Archive and Online tables have the same fields, you can even avoid extra result fields with select expressions like COALESCE(a.field1, b.field1) AS field1
If there are following joins that are dependent on values from ArchiveTable OnlineTable, this can be simplified by performing these core joins in a subquery (at least some coalesces will be necessary though)
SELECT stuff
FROM (
SELECT m.stuff, a.stuff, o.stuff
, COALESCE(a.field1, b.field1) AS xValue
, COALESCE(a.field2, b.field2) AS yValue
, COALESCE(a.field3, b.field3) AS zValue
FROM MainTable AS m
LEFT JOIN ArchiveTable AS a ON #archivedData = 1 AND m.id = a.id
LEFT JOIN OnlineTable AS o ON #archivedData <> 1 AND m.id = o.id
) AS coreQuery
INNER JOIN xTable AS x ON x.something = coreQuery.xValue
INNER JOIN yTable AS y ON y.something = coreQuery.yValue
INNER JOIN zTable AS z ON z.something = coreQuery.zValue
;
If there is criteria narrowing down the MainTable rows to be used, the WHERE for them should be included in the subquery to minimize the amount of Archive/Online carried out of the subquery.
If the Archive/Online table is actually the "main" table, the question's option 3 should work, but I would suggest putting any filtering criteria relevant to those tables in the their UNIONed subqueries.
If there is no filtering criteria on whatever table is "main", I would consider just maintaining two queries (or building one dynamically) so that the subqueries these approaches necessitate are not needed and will not interfere with index use.