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| Methods, apparatus, and articles of manufacture for analyzing memory use |
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Prime implicates and query optimization in relational databases
| Details |
Inventors: Paulley, Glenn Norman; Vorwerk, Kristofer Paul;
Assignee: Sybase, Inc. (Dublin, CA)
Primary Examiner: Mizrahi; Diane D.
Assistant Examiner: Wu; Yicun
Attorney, Agent or Firm: Smart; John A.
An improved normalization methodology enabling better optimization of SQL queries is described. The normalization methodology effectively handles three-valued SQL search conditions and operates in reduced memory environments. A query is examined and selectively normalized by examining each segment of the query to determine whether it is advantageous to fully convert that segment to conjunctive normal form (CNF). If full conversion of the segment to CNF is not advantageous, then only the useful conjunctive conditions (prime implicates) that can be derived from the original input are saved. These useful conjuncts (prime implicates) are unioned with the original statement, resulting in a search condition that is semantically equivalent to the original expression, but that can be better exploited by a query optimizer. This enables better optimization of the original query without the system overhead that would result from full normalization of the query. |
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DETAILED DESCRIPTION The improved normalization methodology of the present invention is based on Socher's minimization methodology (see Socher, R.
, above).
Socher's minimization method uses a matrix as its basic data structure with the rows of the matrix representing unique literals and the columns representing the particular conjunct (or disjunct) in which the literals appear.
Implicates and prime implicates are discovered by generating all paths through the matrix and discarding subsumed paths.
Existing query optimizers require fully normalized search conditions as input and assume the use of two-valued logic.
The improved methodology of the present invention, on the other hand, features several improvements that are key to allowing it to operate with reduced memory requirements on non-normalized three-valued SQL search conditions.
The improved normalization methodology of the present invention comprises a preprocessing phase, in which expressions are simplified whenever possible, and a normalization phase, in which the simplified expression is analyzed and either fully converted to conjunctive normal form ("CNF") or, alternatively, modified by appending prime implicates to the original expression as described below.
In the preprocessing phase, tautological and contradictory conditions are removed from the input expression and other conditions are simplified when possible.
This preprocessing phase includes several steps that are designed to simplify the original query expression, thereby simplifying the matrix processing occurring in the normalization phase.
Pruning conditions from the input data structure reduces memory consumption by decreasing the size of the input to the normalization routines.
The normalization phase takes an unnormalized search condition as input and evaluates the eligibility of the expression or portions of the expression for normalization.
Rather than fully normalizing the expression, which in many cases is inefficient and results in an explosion of terms that must be processed, the normalization phase of the present invention selectively examines each element of the SQL statement by carefully descending the logic tree of the query
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Databases 
