So far we have only discussed query performance, but SQL is not only
about queries. It supports data manipulation as well. The respective
update—form the so-called “data manipulation
language” (DML)—a section of the SQL standard. The performance of these
commands is for the most part negatively influenced by indexes.
An index is pure redundancy. It contains only data that is also stored
in the table. During write operations, the database must keep those
redundancies consistent. Specifically, it means that
update not only affect the table but
also the indexes that hold a copy of the affected data.