# When is a composite database index useful?

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HELP CENTER · FAQ Databases 1 min read
# When is a composite database index useful?
A composite index can speed up queries filtering or sorting on a particular combination of columns, but its usefulness depends on query shape and column order. It consumes storage and adds work to writes, so create it from measured needs.
IaGenify
Databases

## Match index order to query patterns
Many database engines use the leftmost columns of a composite index most effectively. An index on (account_id, created_at) may support filtering by account and ordering dates within it, while a query on created_at alone may not benefit equally.
Review real query plans and representative data. Selectivity, joins, sort order, predicates, and engine behavior all affect whether the index is chosen and reduces work.
## Measure impact and maintain deliberately
Compare execution time, rows scanned, and write overhead before and after adding the index. Building one on a large live table can consume resources or acquire locks; check the engine's online or concurrent options.
Remove redundant indexes only after confirming no important query depends on them. Recheck index usage as data volume and workload change; a once-useful index may become expensive without sufficient benefit. [← Back to Databases FAQs](https://help.iagenify.com/faqs/databases)