In what order should columns be specified in a composite index? Does the order matter?

The order of columns in a composite index is critical for query performance and index utilization.

• The index is built on the sequence of columns, and searching by the index is effective only if the first columns of the index are used in the query.
• For example, for an index (A, B, C), queries filtering by A, or by A and B, or by A, B, and C can efficiently use the index.
• Queries filtering only by B or C (without A) will not be able to effectively use this index.

Therefore, when designing a composite index:

1. First, specify the column with the highest selectivity (most unique, filtering data).
2. Then — columns that are most often used together with the first in WHERE or JOIN conditions.

The correct order allows the most efficient use of the index to speed up queries.