Can a B-tree index be on multiple fields at once?

Yes, a B-tree index can be composite, meaning it is built on multiple columns simultaneously. Such an index is called a composite index and allows for efficient execution of queries that filter or sort data based on multiple columns at once.

For example, in SQL, you can create an index on two columns:

CREATE INDEX idx_name ON table_name (column1, column2);

Within the B-tree structure, the data will be ordered first by column1, and in case of equal values, by column2. This improves the performance of queries with conditions on both columns or on the first column of the index.