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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. 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[1][2]
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For database query optimizers, sargable is an important property in OLTP workloads because it suggests a good query plan can be obtained by a simple heuristic2 matching query to indexes instead of a complex, time-consuming cost-based search,[1] thus it is often desired to write sargable queries. 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
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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.
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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'.
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- ^1 Gulutzan and Pelzer, (Chapter 2, Simple "Searches")
- ^2 [3] gives an example of such simple heuristic.
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