# Functional Indexes in MySQL  ## Metadata - Author: [[Scott Stroz]] - Full Title: Functional Indexes in MySQL - Category: #articles - Summary: MySQL 8.0.13 introduced functional indexes that speed up queries using expressions in the WHERE clause. These indexes work by indexing the result of a function or expression, improving performance without rewriting queries. However, there are rules and limitations to follow when creating and using functional indexes. - URL: https://blogs.oracle.com/mysql/functional-indexes-in-mysql ## Highlights - Functional indexes can increase query performance without having to rewrite the query to address any bottlenecks. ([View Highlight](https://read.readwise.io/read/01m2p1v97x7mvvzcdcvq2rh756)) - • Expressions MUST be contained in parentheses to differentiate them from columns. • `INDEX((col1 + col2))` vs `INDEX( col1, col2)` • We can create an index that has functional and non-functional definitions. • `INDEX((col1 + col2), col1)` • Functional index definitions cannot contain only column names. • `INDEX ((col1), (col2))` will throw an error. • Functional index definitions are not allowed in foreign key columns. • The index will only be used when a query uses the same expression. • `select count(*) from test_data where col1 - col2 = 0` will NOT use the index we created. • We would need to create a new index using the expression `(col1-col2)`. ([View Highlight](https://read.readwise.io/read/01m2p2c417wwt9dgaae0yy3vsx)) - As we have shown, functional indexes can help performance in queries that use expressions in the `WHERE` clause ([View Highlight](https://read.readwise.io/read/01m2p2ck27zye2j8vj93hqvdeq))