What is the difference between two independent indexes, each on one field, and a single combined index containing these two fields?

The difference between two independent indexes on one field and a combined index on two fields lies in how they are used for searching and query optimization.

• Two independent indexes (on separate fields):

  • Each index speeds up searches on its respective field.
  • If a query filters on one of these fields, the index is used efficiently.
  • When filtering on both fields simultaneously, the database can use one of the indexes or perform a union of results, which may be less efficient.

• Combined index (composite index) on two fields:

  • The index stores pairs of values (field1, field2) in a specific order.
  • Very effective for queries filtering on the first field, as well as on both fields together.
  • Less effective for queries filtering only on the second field (without the first), since the index is ordered first by the first field.

Example:

If there is a table with fields first_name and last_name:

• Indexes: INDEX(first_name), INDEX(last_name) — speed up searches on each field separately.
• Combined index: INDEX(first_name, last_name) — speeds up searches on first_name and on the combination of first_name + last_name, but not on last_name alone.

The choice depends on the types of queries that are performed most frequently.