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I have a table with a date column StartDate indexed. I was just wondering how the query optimization phase handles queries of the following form:
SELECT *
FROM <dbo.TABLE>
WHERE (StartDate BETWEEN '2010-01-01' AND '2010-01-10')
OR (StartDate BETWEEN '2010-01-05' AND '2010-01-15')
There is an overlap between the two dates. Should I be doing the optimization myself of giving it the dates as (StartDate BETWEEN '2010-01-01' AND '2010-01-15') or can the SQL engine optimize this by itself?
Compare it with variables and constants: the plans should differ.
SQL Server can optimise for constants but aims for re-use with variables. So the plan is more general with variables. When constants are used, a “generalised reusable plan” isn’t required because if the constants change it will be a new plan
A “variable” plan won’t consider conditions like
end < startwhich would be short circuited with constants.As noted in comments to the question, behaviour is as expected
You should make the same happen for constants with “Forced Parameterization“, but I haven’t tried.