11. 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; identify a first set of candidate queries having prefixes which do not include the prefix of the partial query as a substring and having suffixes including at least three terms in common with the suffix of the partial query; identify a second set of candidate queries having prefixes which do not include the prefix of the partial query as a substring and having suffixes including at least two terms in common with the suffix of the partial query; select the first set of candidate queries as candidate queries for completing the partial query if the first set is not empty, and selecting the second set of candidate queries as the candidate queries for completing the partial query if the first set is empty; and select one or more terms of unique suffixes among the suffixes of the candidate queries as suggested query completions for the partial query.
11. 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; identify a first set of candidate queries having prefixes which do not include the prefix of the partial query as a substring and having suffixes including at least three terms in common with the suffix of the partial query; identify a second set of candidate queries having prefixes which do not include the prefix of the partial query as a substring and having suffixes including at least two terms in common with the suffix of the partial query; select the first set of candidate queries as candidate queries for completing the partial query if the first set is not empty, and selecting the second set of candidate queries as the candidate queries for completing the partial query if the first set is empty; and select one or more terms of unique suffixes among the suffixes of the candidate queries as suggested query completions for the partial query. 14. The system of claim 11 , wherein the instructions further include instructions to 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.