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Composite indexes are created on multiple columns of a table and allow for faster queries that filter or sort data based on these columns.

Basic principles for creating composite indexes:

• The order of columns is important. The index is effective if queries use the columns in the same order as specified in the index.
• Frequently used and selective columns are placed first. This increases search efficiency.
• Do not include too many columns. This increases the size of the index and slows down insert/update operations.
• The index should match typical queries. Analyze which columns are most frequently filtered and sorted.

Example of creating a composite index in SQL:

CREATE INDEX idx_user_lastname_firstname ON users (lastname, firstname);

This index will be useful for queries where filtering or sorting is first done by lastname, then by firstname.