2012-04-18Math

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.

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)