Does the order of attributes in a PostgreSQL composite index matter?

Yes, the order of attributes in a composite index in PostgreSQL matters and affects query performance.

A composite index is created on multiple columns, for example (column1, column2). Such an index is optimally used for queries that filter or sort data first by column1, and then by column2.

Why the order is important:

• The index is ordered first by the first column, then by the second, and so on.
• Queries that use the first columns of the index in their filter or sort conditions can efficiently utilize the index.
• If a query filters only by the second column, the (column1, column2) index will not be used efficiently.

Example:

• The index (last_name, first_name) is well-suited for queries filtering by last_name or both last_name and first_name.
• A query filtering only by first_name will not be able to use this index efficiently.

Therefore, when designing composite indexes, it is important to consider which columns are most frequently used in WHERE conditions and in what order.