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Do You Need a SQL Server Query Hint? However, in those versions there was such a small number of hints you could count them with your fingers. Today, if you look at SQL Server’s documentation, there’s not just one page that lists all the hints because there are too many. You can go to the Hints (Transact- SQL) page to see a list of the three different types of hints, but then you have to go to the page for each type of hint to see a list. In the English language a hint is a gentle suggestion, but in SQL Server a hint is a directive. A hint tells the SQL Server optimizer what should be in the query plan and the optimizer will do it, unless the hint is impossible to implement.
In fact, many people talk about using hints as forcing a query plan. The optimizer determines what indexes will be used, the order in which the table will be processed, the kind of join algorithm that will be executed, whether the query will run on multiple processors, etc. It is probably one of the most complex parts of the entire engine. In early versions of SQL Server, the engineers writing the optimizer thought that one day they would have an optimizer that would always come up with the absolute best plan for every query and that hints would no longer be needed. In fact, prior to SQL Server 7, any time I was able to use a hint to come up with a better query plan than the optimizer, the optimizer engineers wanted to know about it. They wanted to try to figure out why the optimizer didn’t come up with that plan without a hint.
As more features were added to SQL Server and queries became more and more complex on larger data sets, the optimizer is now so complex and has so many possible plans to investigate there’s no way it could always come up with the best plan. The goal now is to come up with a “good enough plan” without taking more time to optimize than the query will need for execution. I actually call the second type “option hints” because they’re specified in an OPTION clause at the end of your queries (and I think all hints are query hints). There’s a join hint to force a LOOP join and an option hint called LOOP JOIN, and you might be wondering what the difference is. The join hint is specified in the JOIN clause (so you must be using the join specification using the JOIN keyword). When you use a join hint, it applies to only the two tables on either side of the JOIN keyword. If you use LOOP JOIN as an option hint, it applies to all the joins in the query.
So you might think that it doesn’t matter whether you use a join hint or an option hint if there are only two tables, but think again. If you use a join hint, it has the side effect of forcing the join order between the tables. In the first query below, in addition to using a LOOP JOIN the optimizer will make sure that Sales. Order. Header is the first table accessed during execution, whereas in the second query, the optimizer can decide for itself which table should be accessed first. If the optimizer is upgraded in a service pack, you might never know it. And worse, if your data changes so that the plan forced by your hints is no longer the best plan, you might start getting worse performance than you would have without the hint!
However, that place should not be at the top of your list of query tuning techniques, it should be much closer to the bottom of the list. Hints can be a solution if you haven’t been able to find any other way to get SQL Server’s optimizer to find an acceptable plan. But take a hint from me and learn all you can about tuning and optimization before you start liberally sprinkling query hints throughout your code.
Is there a tool out there which can convert SQL syntax to LINQ syntax? I just want to rewrite basic queries with join, etc., to LINQ. It would save me a lot of time.
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