by Markus Winand.

Math


There is one more class of obfuscations that is smart and prevents proper index usage. Instead of using logic expressions it is using a calculation.

Consider the following statement. Can it use an index on NUMERIC_NUMBER?

SELECT numeric_number
  FROM table_name
 WHERE numeric_number - 1000 > ?

Similarly, can the following statement use an index on A and B—you choose the order?

SELECT a, b
  FROM table_name
 WHERE 3*a + 5 = b

Let's put these questions into a different perspective; if you were developing an SQL database, would you add an equation solver? Most database vendors just say “No!” and thus, neither of the two examples uses the index.

On my Own Behalf

If you like this article, you might also like my book SQL Performance Explained or my training.

You can even use math to obfuscate a condition intentionally—as we did it previously for the full text LIKE search. It is enough to add zero, for example:

SELECT numeric_number
  FROM table_name
 WHERE numeric_number + 0 = ?

Nevertheless we can index these expressions with a function-based index if we use calculations in a smart way and transform the where clause like an equation:

SELECT a, b
  FROM table_name
 WHERE 3*a - b = -5

We just moved the table references to the one side and the constants to the other. We can then create a function-based index for the left hand side of the equation:

CREATE INDEX math ON table_name (3*a - b)

About the Author

Photo of Markus Winand

Markus Winand teaches efficient SQL—inhouse and online. He minimizes the development time using modern SQL and optimizes the runtime with smart indexing. His book entitled SQL Performance Explained has become standard reading.

Buy his Book on Amazon

Cover of “SQL Performance Explained”: Squirrel running on grass

The essence of SQL tuning in 200 pages

Buy on Amazon
(paperback only)

Paperback and PDF also available at Markus’ store.

Hire Markus

…to answer your current SQL questions.

The quick and easy way to benefit from his extensive knowledge and experience.
Learn more »

“Use The Index, Luke!” by Markus Winand is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 Unported License.
Legal | Contact | NO WARRANTY | Trademarks | Privacy | CC-BY-NC-ND 3.0 license