How important is the order of columns in indexes?
Indexes are essential tools for optimizing database queries, but understanding the importance of the order of columns in multi-column indexes is equally crucial. In PostgreSQL (and most relational databases), the order of columns in an index can dramatically impact query performance, especially for complex queries involving WHERE clauses, sorting, and joins.
This article explores why the order of columns in an index matters and how to determine the best column order for your use case.
1. How Multi-Column Indexes Work
A multi-column index (also called a composite index) in PostgreSQL stores data in a sorted order based on the column sequence defined during index creation. For example, if you create an index on columns (A, B), the index is ordered first by column A, and then within each value of A, it is ordered by column B.
The following index:
CREATE INDEX idx_example ON my_table (A, B);Is similar to creating two separate indexes, but instead of indexing A and B independently, it first orders by A and then by B. This order is vital for efficient query lookups, especially for query conditions involving both columns.
