Data

Leftmost Prefix Rule

A composite index on (a, b, c) can be seeked by a, or a+b, or a+b+c - but never by b alone.

Why interviewers ask about it

Explains why one well-ordered composite index beats three single-column ones. Put equality columns before the range column.

An index containing every column a query needs, so the table is never read.

This term is part of the free System Design interview preparation module - browse the full glossary for every definition.