9. A system, comprising: a data processing apparatus including a hardware processor; and a non-transitory computer readable storage medium storing software instructions executable by the data processing apparatus and that upon such execution cause the data processing apparatus to perform operations comprising: for each query submitted for a respective first search operation and for which first resources have been identified by the first search operation as being responsive the query, wherein each first resource is a web-page resource: determining a search probability ratio for the query that is at least based on a measure of a likelihood of the query being submitted for a second search operation being of search operation type that is different from a search operation type of the first search operation; for each query for which the search probability ratio for the query meets a threshold search probability ratio: initiating the second search operation and receiving data indicating second resources identified by the second search operation as being responsive the query, each second resource being a native application; determining, based at least on the search probability ratio, whether to insert a search result identifying the second resource in a set of search results identifying the first resources; and for each query for which a determination is made to insert a search result identifying the second resource in a set of search results identifying the first resources, inserting the search result identifying the second resource in the set of search results identifying the first resources.
9. A system, comprising: a data processing apparatus including a hardware processor; and a non-transitory computer readable storage medium storing software instructions executable by the data processing apparatus and that upon such execution cause the data processing apparatus to perform operations comprising: for each query submitted for a respective first search operation and for which first resources have been identified by the first search operation as being responsive the query, wherein each first resource is a web-page resource: determining a search probability ratio for the query that is at least based on a measure of a likelihood of the query being submitted for a second search operation being of search operation type that is different from a search operation type of the first search operation; for each query for which the search probability ratio for the query meets a threshold search probability ratio: initiating the second search operation and receiving data indicating second resources identified by the second search operation as being responsive the query, each second resource being a native application; determining, based at least on the search probability ratio, whether to insert a search result identifying the second resource in a set of search results identifying the first resources; and for each query for which a determination is made to insert a search result identifying the second resource in a set of search results identifying the first resources, inserting the search result identifying the second resource in the set of search results identifying the first resources. 10. The system of claim 9 , wherein the initiating the second search operation and determining whether to insert a search result occurs only when the search probability ratio meets a threshold search probability ratio.