17. The system of claim 13 , wherein said means for automatically rewriting includes said means for performing said join-back algorithm, and wherein said means for performing said join-back algorithm further includes: means for ordering D 1 to D n in ascending selectivity order of Si, wherein said user query (Q) is of a form σ s R JC1 σ S1 D 1 . . . JCn σ Sn D n in which said user query Q is said σ s R, and a relational table R joins each table D i using condition JCi; means for generating n+1 SQL queries said means for generating said n+1 SQL queries Q i including means for pushing from 0 to n semi-joins in an order specified by said ordering; means for compiling said n+1 SQL queries Q[i], said means for compiling including means for utilizing an optimizer of said DBMS; means for determining, by said DBMS, estimates of costs of computing resources required for an execution of each SQL query Q[i]; and means for selecting, as said rewritten query and subsequent to said determining, an SQL query Q j of said n+1 SQL queries Q[i], said SQL query Q j being associated with a least estimate of said determined estimates.
17. The system of claim 13 , wherein said means for automatically rewriting includes said means for performing said join-back algorithm, and wherein said means for performing said join-back algorithm further includes: means for ordering D 1 to D n in ascending selectivity order of Si, wherein said user query (Q) is of a form σ s R JC1 σ S1 D 1 . . . JCn σ Sn D n in which said user query Q is said σ s R, and a relational table R joins each table D i using condition JCi; means for generating n+1 SQL queries said means for generating said n+1 SQL queries Q i including means for pushing from 0 to n semi-joins in an order specified by said ordering; means for compiling said n+1 SQL queries Q[i], said means for compiling including means for utilizing an optimizer of said DBMS; means for determining, by said DBMS, estimates of costs of computing resources required for an execution of each SQL query Q[i]; and means for selecting, as said rewritten query and subsequent to said determining, an SQL query Q j of said n+1 SQL queries Q[i], said SQL query Q j being associated with a least estimate of said determined estimates. 18. The system of claim 17 , wherein said means for generating said n+1 SQL queries Q[i] comprises: means for setting a counter to an initial value; and means for performing a loop for said counter less than or equal to a terminal value, said means for performing said loop including: means for setting T to a first projection Π ckey (σ s R) if said counter is equal to said initial value, otherwise for setting T to a second projection Π ckey (σ s R JCi σ Si D 1 . . . JCi σ Si D i ), wherein said first projection and said second projection project input data of said first projection and said second projection, respectively, on ckey and remove duplicates therein, said ckey being a cluster key included in a definition of cleansing rule C, and wherein said setting T to said first projection and setting T to said second projection facilitate said pushing; means for generating Q[i] as an expression σ s′ ((Φ c (σ ec (R) ckey (T))), wherein ec is an expanded condition and s′ is an optimized query condition, wherein said Φ c (σ ec (R) ckey (T)) is a result of applying said cleansing rule C on a data set σ ec (R), wherein said data set σ ec (R) is a result of directly pushing said expanded condition ec to said relational table R and cleansing data of said relational table R selected by said expanded condition ec, and wherein said means for generating includes: means for computing said expanded condition ec as s∥cc, wherein said s is a query condition on said relational table R and is included in said user query Q, and wherein said cc is a context condition defining a context set for a context reference X, said context reference X included in one or more context references included in a pattern of said cleansing rule C, and means for simplifying said query condition s to said optimized query condition s′, said means for simplifying including means for setting said optimized query condition s′ equal to s−cc; and means for incrementing said counter.