PostgreSQL: Creating Indexes on Expressions (Functional Indexes)

Optimize queries by creating indexes on expressions in PostgreSQL. This guide explains how functional indexes can improve query performance.

Creating Indexes on Expressions in PostgreSQL

Introduction

PostgreSQL allows you to create indexes not only on individual columns but also on expressions involving columns. These are called indexes on expressions or functional indexes. They improve query performance when your queries use those expressions in their WHERE or ORDER BY clauses.

Understanding Indexes on Expressions

A regular index is created on one or more columns of a table. An index on an expression allows you to index the *result* of an expression, rather than just the column values themselves. This can be particularly helpful for optimizing queries that use functions or expressions in their WHERE clauses.

Syntax for Creating an Index on an Expression

CREATE INDEX index_name ON table_name (expression);

• index_name: The name you give to the new index.
• table_name: The table the index applies to.
• expression: The expression to index (e.g., a function call involving table columns).

Important Note: Maintaining indexes on expressions can be computationally expensive because PostgreSQL must evaluate the expression for every row. Indexes on expressions are most beneficial when the speed of retrieving data is more important than the speed of inserting or updating data.

Example: Case-Insensitive Search Optimization

Let's say you have an `employees` table with an `emp_lname` column and you frequently search for employees by their last name (case-insensitive). A regular index on `emp_lname` won't help with case-insensitive searches.

(Example showing a query on the `employees` table would be included here. The example should demonstrate how an index on `emp_lname` is used effectively when the query uses `emp_lname = 'Brown'`. But if the query uses `UPPER(emp_lname) = 'Brown'`, the index is not used.)

To optimize case-insensitive searches, create an index on the uppercase version of the last name:

CREATE INDEX idx_emp_lname_upper ON employees (UPPER(emp_lname));

(Example showing how the query plan changes and uses `idx_emp_lname_upper` when searching using `UPPER(emp_lname)` would be added here.)

Conclusion

Indexes on expressions are a powerful technique for optimizing queries that involve functions or expressions. By strategically indexing the results of these expressions, you can significantly improve query performance in PostgreSQL.