16. The system of claim 15 , the operations further comprising: determining that the one or more entities of the second entity type have the predetermined relationship with the first entity to which the first search query relates, comprising determining that a related entity index maps the first entity to the one or more entities of the second entity type.
16. The system of claim 15 , the operations further comprising: determining that the one or more entities of the second entity type have the predetermined relationship with the first entity to which the first search query relates, comprising determining that a related entity index maps the first entity to the one or more entities of the second entity type. 17. The system of claim 16 , the operations further comprising: obtaining data identifying the first entity and the first entity type; obtaining data identifying the one or more entities of the second entity type and the relationship between the first entity and the one or more entities of the second entity type; and generating a mapping between the first entity and the one or more entities of the second entity type in the related entity index.