Sargable
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. It was first used by IBM researchers as a contraction of Search ARGument, and has come to mean simply "can be looked up by an index."1
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.
Some database management systems, for instance PostgreSQL, support functional indices. Conceptually, an index is simply a mapping between a value and one or more locations. With a functional index, the value stored in the index is the output of the function specified when the index is created. This capability expands what is sargable beyond base column expressions.
- Sargable operators:
=, >, <, >=, <=, BETWEEN, LIKE, IS [NOT] NULL, IN - Sargable operators that rarely improve performance:
<>, NOT, 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 SQRT(myIntField) > 11.7
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.
Text example
WHERE ... LIKE clauses that are sargable have field values on the left of the operator, and LIKE text strings that do not begin with the % on the right.
Not sargable:
SELECT *
FROM myTable
WHERE myNameField LIKE '%Wales%' -- Begins with %, not sargable
This is not sargable. It must examine every row to find the fields containing the substring 'Wales' in any position.
Sargable version:
SELECT *
FROM myTable
WHERE myNameField LIKE 'Jimmy%' -- Does not begin with %, sargable
This is sargable. It can use an index to find all the myNameField values that start with the substring 'Jimmy'.
See also
Notes
- ^1 Gulutzan and Pelzer, (Chapter 2, Simple "Searches")
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)