Algorithms & Data Structures·Published Aug 4, 2026·Updated Aug 10, 2026

MySQL Indexes: From a Slow Query to a Better B+ Tree

Tracing a slow query through execution plans, composite indexes, and access patterns, with a look at the write and storage costs behind faster reads.

MySQL Indexing Performance

Indexes are not magic switches. They are data structures with tradeoffs, and the useful habit is learning how a query reaches the rows it needs.

Start with the query

Before adding an index, inspect the filter, sort, join, and selected columns. A composite index should match how the database narrows the search.

Read the execution plan

EXPLAIN helps show whether MySQL is scanning too much data, using the intended index, or paying for extra sorting.

Tradeoffs

Indexes speed up reads but add write cost and storage. The best index is tied to a real access pattern, not just a column that looks important.