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Avoid joining tables on a range in SQL Server

An example of a query to avoid: Even when columns are indexed and the result set is only one row, SQL Server can choke.

Even when columns are indexed and the result set returned is only one row, SQL Server can choke. In my case, analyzing the query showed that it was attempting to scan the entire table for both the ">" selection and the "<" selection. Converting these to sub-selects did nothing to prevent the full table scans. Watch out for this!

 

 SELECT t1.col1, t1.col2 t1.col3, t2.col1, t2.col2, t3.col3 FROM table1 t1, table2 t2 WHERE t1.col1 = 123456 AND t1.col2 > t2.col2 AND t1.col3 < t2.col3

 

Feedback About this Tip

Shawn H. points out that "the reason the query table scanned was probably because of the > t2.col3. If an index existed on that column you would need to modify the query to a >= t2.col3 or add an index and do the same. I would be willing to bet that this is the point where the server decided a table scan was the query plan of choice because > t2.col3 is not giving it a starting point in an index therefore the query optimizer decides a table scan is in order. This query also is not doing a range. Every column in the WHERE is different. A range would have been as follows:

 SELECT t1.col1, t1.col2 t1.col3, t2.col1, t2.col2, t3.col3 FROM table1 t1, table2 t2 WHERE t1.col1 = 123456 AND t1.col2 > t2.col2 this should be t1.col2 >= t2.col2 AND t1.col2 < t2.col2

 

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