Even if none of these columns were indexed, our expectation would be a table scan on Product and a table scan on SalesOrderDetail. The query is simple enough: 2 tables and a join that checks both ProductID and rowguid. Here is a very simple example of how an OR can cause performance to become far worse than you’d ever imagine it could be: Even if only a few tables or columns are involved, the performance can become mind-bogglingly bad. We not only need to evaluate each component of the OR clause, but need to follow that path through the other filters and tables within the query. The scenario in which OR performs worst is when multiple columns or tables are involved. When this expensive operation is completed, the results can then be concatenated and returned normally. Instead, each component of the OR must be evaluated independently. Because it is inclusive, SQL Server cannot process it in a single operation. By being exclusive, these operations take data and slice it into progressively smaller pieces, until only our result set remains. SQL Server can efficiently filter a data set using indexes via the WHERE clause or any combination of filters that are separated by an AND operator. Tips and tricks OR in the Join Predicate/WHERE Clause Across Multiple Columns While we should perform our due diligence and prove that any changes we make are optimal, knowing where to start can be a huge time saver!įor more information about Query optimization, see the SQL Query Optimization - How to Determine When and If It’s Needed article Whereas performance tuning can often be composed of hours of collecting extended events, traces, execution plans, and statistics, being able to identify potential pitfalls quickly can short-circuit all of that work. Sometimes we can quickly cut that time by identifying common design patterns that are indicative of poorly performing TSQL.ĭeveloping pattern recognition for these easy-to-spot eyesores can allow us to immediately focus on what is most likely to the problem. Fixing bad queries and resolving performance problems can involve hours (or days) of research and testing.
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