23. A system including memory and one or more processors operable to execute instructions, stored in the memory, to select suggested query completions for a partial query, including instructions to: determine a prefix and a suffix of a partial query having multiple terms, the prefix including one or more terms occurring at a beginning of the partial query, the suffix including one or more terms occurring at an end of the partial query, wherein a last term in the prefix precedes a first term in the suffix; calculate suffix similarity scores for unique queries having prefixes which do not include the prefix of the partial query as a substring, the suffix similarity score for a unique query based at least in part on similarity of terms in the suffix of the partial query and terms in a suffix of the unique query; select one or more of the unique queries as candidate queries for completing the partial query based at least in part on the suffix similarity scores for the unique queries; calculate completion scores for unique suffixes among the suffixes of the candidate queries, the completion scores representing an extent to which the unique suffixes are potential query completions for the partial query; and use the completion scores to select one or more terms of the unique suffixes as suggested query completions for the partial query.
23. A system including memory and one or more processors operable to execute instructions, stored in the memory, to select suggested query completions for a partial query, including instructions to: determine a prefix and a suffix of a partial query having multiple terms, the prefix including one or more terms occurring at a beginning of the partial query, the suffix including one or more terms occurring at an end of the partial query, wherein a last term in the prefix precedes a first term in the suffix; calculate suffix similarity scores for unique queries having prefixes which do not include the prefix of the partial query as a substring, the suffix similarity score for a unique query based at least in part on similarity of terms in the suffix of the partial query and terms in a suffix of the unique query; select one or more of the unique queries as candidate queries for completing the partial query based at least in part on the suffix similarity scores for the unique queries; calculate completion scores for unique suffixes among the suffixes of the candidate queries, the completion scores representing an extent to which the unique suffixes are potential query completions for the partial query; and use the completion scores to select one or more terms of the unique suffixes as suggested query completions for the partial query. 33. The system of claim 23 , including instructions to send one or more of the selected terms of the unique suffixes in response to receiving the partial query.