Sargable
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In relational databases, a condition (or predicate) in a query is said to be sargable if the DBMS engine can take advantage of an index to speed up the execution of the query. The term is derived from a contraction of Search ARGument ABLE.
A query failing to be sargable is known as a non-sargable query and typically has a negative effect on query time, so one of the steps in query optimization is to convert them to be sargable. The effect is similar to searching for a specific term in a book that has no index, beginning at page one each time, instead of jumping to a list of specific pages identified in an index.
The typical situation that will make a SQL query non-sargable is to include in the WHERE clause a function operating on a column value. The WHERE clause is not the only clause where sargability can matter; it can also have an effect on ORDER BY, GROUP BY and HAVING clauses. The SELECT clause, on the other hand, can contain non-sargable expressions without adversely affecting the performance.
- Sargable operators:
=, >, <, >=, <=, BETWEEN, LIKE, IS [NOT] NULL - Sargable operators that rarely improve performance:
<>, IN, OR, NOT IN, NOT LIKE
Simple example
WHERE clauses that are sargable typically have field values on the left of the operator, and scalar values or expressions on the right side of the operator.
Not sargable:
SELECT *
FROM myTable
WHERE 11.7 < SQRT(myIntField)
This is not sargable because myIntField is embedded in a function. If any indexes were available on myIntField, they could not be used. In addition, SQRT() would be called on every record in myTable.
Sargable version:
SELECT *
FROM myTable
WHERE myIntField > 11.7 * 11.7
This is sargable because myIntField is NOT contained in a function, making any available indexes on myIntField potentially usable. Furthermore, the expression is evaluated only once, rather than for each record in the table.
See also
References
- SQL Performance Tuning by Peter Gulutzan, Trudy Pelzer (Addison Wesley, 2002) ISBN 0-201-79169-2 (Chapter 2, Simple "Searches")
- Microsoft SQL Server 2012 Internals by Kalen Delaney, Connor Cunningham, Jonathan Kehayias, Benjamin Nevarez, Paul S. Randal (O'Reily, 2013) ISBN 978-0-7356-5856-1 (Chapter 11, The Query Optimizer)