How Do Composite Indexes Work to Speed Up Database Lookups?

How do Composite Indexes Function?

Composite indexes, often referred to as multicolumn indexes, provide a means to improve database performance by enabling rapid lookups based on multiple fields. Understanding how they operate is crucial for optimizing database queries.

Composite Index Mechanism

Your assumption regarding the ordering of columns in a composite index is generally correct. The specified order defines the grouping of values in the index. For instance, if the index is defined as (a ASC, b ASC, c ASC), the records will be sorted initially by the values in column 'a', then by column 'b', and finally by column 'c'. This arrangement creates multiple indexes, effectively grouping records with identical 'a' values together.

Example

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 a composite index is created as (a ASC, b ASC, c ASC), the index will be structured as follows:

[Index on 'a']

1 -> {[[1 | 2 | 3], [1 | 4 | 2], [1 | 4 | 4]], [[2 | 3 | 5], [2 | 4 | 4], [2 | 4 | 5]]}

[Index on 'b' within group of equal 'a']

1 -> {[[1 | 2 | 3], [1 | 4 | 2]], [[1 | 4 | 4]]}
2 -> {[[2 | 3 | 5]], [[2 | 4 | 4], [2 | 4 | 5]]}

[Index on 'c' within group of equal 'a' and 'b']

1 -> 2 -> 3
1 -> 4 -> 2
1 -> 4 -> 4
2 -> 3 -> 5
2 -> 4 -> 4
2 -> 4 -> 5

This structure effectively creates multiple indexes, each storing data grouped by 'a' values. For a given 'a' value, the 'b' values are further sorted, followed by the sorting of 'c' values within each 'a' and 'b' group.

By utilizing these multi-valued keys, composite indexes provide efficient access to records based on the specified order of fields. They significantly enhance search performance when queries involve searching for specific combinations of field values.

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