How does the placement and order of columns in an index affect its efficiency and database performance?

The order and placement of columns in a composite index critically affect query efficiency and database performance. The index is built as a tree (for example, a B-tree), where sorting occurs first by the first column, then by the second, and so on.

If a query filters or sorts data by the first column of the index, the index will be most effective. If the query only uses the second or subsequent columns, the index may not be used or may be used partially.

For example, for an index on columns (A, B, C):

• A query with a condition on A or on A and B will use the index efficiently.
• A query only on B or C — the index will not be fully utilized.

Therefore, when designing indexes, it is important to place the most selective and frequently used columns in filters at the front. This will improve query performance and reduce the load on the database.