Is the order of fields in a composite index important?

Yes, the order of fields in a composite index is very important. In relational databases, a composite index is created based on the sequence of columns, and this sequence affects which queries will efficiently utilize the index.

For example, if you have an index on fields (A, B), it is effectively used for queries filtering by A or by both A and B together, but not for queries filtering only by B.

This is because the index is structured as a tree, sorted first by the first field, then by the second, and so on. Therefore, it is important to consider the order of fields in the index based on which queries are most frequent and critical for performance.