Amazon Aurora DSQL: The Definitive Guide

Appendix B. Multi-Column Indexes Versus Covering Indexes

A Note for Early Release Readers

With Early Release ebooks, you get books in their earliest form—the author’s raw and unedited content as they write—so you can take advantage of these technologies long before the official release of these titles.

The GitHub repo is active. If you’d like to be actively involved in reviewing and commenting on this draft, please reach out to the editor at [e-mail].

These two concepts are frequently confused, especially because they can be combined. This appendix clarifies what each one does, when to use which, and how they interact — with DSQL-specific guidance on the trade-offs.

The Core Distinction

A multi-column index puts multiple columns in the index’s key — the sorted structure the database traverses during lookups. All key columns participate in the sort order and can be used in WHERE, ORDER BY, and join predicates.

A covering index adds extra columns as payload via INCLUDE. These columns are stored in the index leaf pages but are not part of the sort order. They exist solely so the database can answer a query without touching the heap (data table).

Property	Multi-column key	INCLUDE (covering) columns
In sort order	Yes — determines how the B-tree is organized	No — stored in leaf pages only
Usable in WHERE/ORDER BY	Yes	No
Usable in join predicates	Yes	No
Stored in internal tree nodes	Yes — adds overhead to tree navigation	No — leaf pages only
Available for ...