YugabyteDB provides consistent, high-performance secondary indexes built on top of distributed ACID transactions. Secondary indexes can be created using one or more columns of a database table and provide the basis for both rapid random lookups and efficient access of ordered records when querying by those columns. Creating a secondary index requires additional writes and storage space to maintain the index data structure. YugabyteDB uses distributed ACID transactions under the hood in order to maintain consistency of secondary indexes, ensuring that read queries see consistent values even when updates are happening at the same time as reads. Secondary indexes can be used to efficiently look up users by their email address or other columns, and can be used with pre-existing data, allowing for future flexibility.