# Join Order

A Join is a predicate that attempts to combine 2 row sources. We only ever join 2 row sources together. Join steps are always performed serially even though underlying row sources may have been accessed in parallel. The join order makes a significant difference to the way in which the query is executed. By accessing particular row sources first, certain predicates may be satisfied that are not satisfied by with other join orders. This may prevent certain access paths from being taken.

- Make sure the join between 2 tables is done via indexed fields as much as possible.
- Also, if such an index exists, make sure you specify fields in the order they are defined by that index.

Parent topic: Common Issues to Be Aware of