How Do Composite Indexes Really Work: Is Column Order the Key to Grouping?

Delving into the Functionality of Composite Indexes

Composite indexes enhance performance by facilitating efficient data retrieval for tables with multiple fields. Understanding their inner workings is crucial for optimizing database performance.

The Myth and the Reality

A common assumption is that the sequence of columns specified in the index definition determines the grouping order. However, this is incorrect.

The Truth Unveiled

In a composite index, the records are primarily sorted based on the values in the first column (a). The index is then subdivided into groups based on the values in the second column (b). Each group is further subdivided into subgroups based on the values in the third column (c).

Visualizing the Structure

Consider a composite index on columns (a,b,c):

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| a1 | b1 | c1 | a1 | b2 | c2 | a1 | b3 | c3 |
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|  1  |  2  |  3  |  1  |  4  |  2  |  1  |  4  |  4  |
--------------------------------------------------

Here, the index is primarily sorted by 'a' values. Within each 'a' group, the records are sorted by 'b' values. Finally, within each 'b' subgroup, the records are sorted by 'c' values.

Conclusion

Composite indexes enhance performance by creating multi-level sorting mechanisms. They prioritize data retrieval by ordering records based on the specified columns, but unlike a simple assumption, the grouping is not directly based on the column order. Understanding this nuanced functionality enables database administrators to optimize indexes for faster and more efficient queries.

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