17. One or more computer-readable non-transitory storage media embodying software operable when executed by one or more computer systems to: select from a set of query-suggestion pairs a first query, wherein: each query-suggestion pair from the set of query-suggestion pairs comprises a query and a suggestion; and the first query is the query of at least one query-suggestion pair from the set of query-suggestion pairs; select from the set of query-suggestion pairs a subset of query-suggestion pairs, wherein: the subset of query-suggestion pairs comprises one or more distinct query-suggestion pairs from the set of query-suggestion pairs; and the first query is the query of each query-suggestion pair from the subset of query-suggestion pairs; compute a Log Likelihood Ratio (LLR) value for each query-suggestion pair from the subset of query-suggestion pairs; rank the subset of query-suggestion pairs according to their respective LLR values; remove from the subset of query-suggestion pairs all query-suggestion pairs whose LLR values are below a predetermined LLR threshold; compute a Pointwise Mutual Information (PMI) value for each remaining query suggestion pair from the subset of query-suggestion pairs; remove from the subset of query-suggestion pairs all query-suggestion pairs whose PMI values are below a predetermine PMI threshold; and construct a ranked set of suggestions for the first query, wherein the ranked set of suggestions comprises one or more suggestions of the remaining query-suggestion pairs from the subset of query-suggestion pairs.
17. One or more computer-readable non-transitory storage media embodying software operable when executed by one or more computer systems to: select from a set of query-suggestion pairs a first query, wherein: each query-suggestion pair from the set of query-suggestion pairs comprises a query and a suggestion; and the first query is the query of at least one query-suggestion pair from the set of query-suggestion pairs; select from the set of query-suggestion pairs a subset of query-suggestion pairs, wherein: the subset of query-suggestion pairs comprises one or more distinct query-suggestion pairs from the set of query-suggestion pairs; and the first query is the query of each query-suggestion pair from the subset of query-suggestion pairs; compute a Log Likelihood Ratio (LLR) value for each query-suggestion pair from the subset of query-suggestion pairs; rank the subset of query-suggestion pairs according to their respective LLR values; remove from the subset of query-suggestion pairs all query-suggestion pairs whose LLR values are below a predetermined LLR threshold; compute a Pointwise Mutual Information (PMI) value for each remaining query suggestion pair from the subset of query-suggestion pairs; remove from the subset of query-suggestion pairs all query-suggestion pairs whose PMI values are below a predetermine PMI threshold; and construct a ranked set of suggestions for the first query, wherein the ranked set of suggestions comprises one or more suggestions of the remaining query-suggestion pairs from the subset of query-suggestion pairs. 18. The media recited in claim 17 , wherein the software is further operable when executed by one or more computer systems to: extract from one or more logs one or more query sequences, wherein each query sequence comprises one or more queries received at a search engine ordered according to when the one or more queries are received at the search engine; construct one or more query-suggestion pairs to be included in the set of query-suggestion pairs from each query sequence, wherein constructing one query-suggestion pair from one query sequence comprises: select from the one query sequence two queries; and construct the one query-suggestion pair with a preceding one of the two queries as the query and a succeeding one of the two queries as the suggestion.