| Operator | Points |
|---|---|
| = | 10 |
| > | 5 |
| >= | 5 |
| < | 5 |
| <= | 5 |
| LIKE | 3 |
| <> | 0 |
| Operand | Points |
|---|---|
| Literal alone | 10 |
| Column alone | 5 |
| Parameter alone | 5 |
| Multioperand expression | 3 |
| Exact numeric data type | 2 |
| Other numeric data type | 1 |
| Temporal data type | 1 |
| Character data type | 0 |
| NULL | 0 |
The left side of a search condition should be a simple column
name; the right side should be an easy-to-look-up value. Each component of a search condition has a point count. The
higher the points, the faster the component. The condition that takes the least
time gets the most points. Put multiple expressions in the correct order. Use the Law of Transitivity and the concept of constant
propagation to substitute a literal for a column name or column expression
whenever you can do so without changing the meaning of an expression. Some DBMSs won't fold most obvious-looking (to a C programmer)
expressions. Don't use this principle as the reason to always transform. such
expressions when you find them in old code though—usually they're there for
historical reasons. Remember Rule #1: Understand the code before changing
it. If the code involves an obvious math expression, do evaluate it
and transform. the condition to the evaluated result. Constant folding can lead
to constant propagation and is therefore A Good Thing. Avoid functions on columns. If you can't avoid functions, don't use UPPER to ensure case
insensitivity. Use LOWER insteadThe Bottom Line: General Tuning
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