How Do Composite Indexes Actually Organize Database Data?

Understanding Composite Indexes

Composite indexes are a valuable tool for optimizing database performance by organizing data efficiently. They allow you to index multiple columns simultaneously, enabling faster data retrieval based on those columns.

How Composite Indexes Function

Contrary to your assumption, composite indexes do not group data based on the order of column specification. Instead, they sort records based on the specified sort order for each individual column.

For example, consider a composite index on columns a, b, and c. If you specify the index as a ASC, b ASC, c ASC, it means that:

• Records are sorted in ascending order by column a.
• Within each group of records with the same a value, records are further sorted in ascending order by column b.
• Within each group of records with the same a and b values, records are finally sorted in ascending order by column c.

Therefore, the resultant index is effectively a multi-valued key where each key consists of the values from the three indexed columns.

Example

Let's consider the following table:

| A | B | C |

| 1 | 2 | 3 |
| 1 | 4 | 2 |
| 1 | 4 | 4 |
| 2 | 3 | 5 |
| 2 | 4 | 4 |
| 2 | 4 | 5 |

If you create a composite index on (a, b, c), the index will be structured as follows:

[(1, 2, 3), (1, 4, 2), (1, 4, 4), (2, 3, 5), (2, 4, 4), (2, 4, 5)]

By utilizing composite indexes, you can efficiently query data based on multiple columns, reducing the need for full table scans and improving performance.

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