Query processing and optimization (updated)
Download as PPTX, PDF15 likes27,775 views
The document discusses techniques used by a database management system (DBMS) to process, optimize, and execute high-level queries. It describes the phases of query processing which include syntax checking, translating the SQL query into an algebraic expression, optimization to choose an efficient execution plan, and running the optimized plan. Query optimization aims to minimize resources like disk I/O and CPU time by selecting the best execution strategy. Techniques for optimization include heuristic rules, cost-based methods, and semantic query optimization using constraints.
Query processing and optimization (updated)
- 2. Introduction • In this chapter we shall discuss the techniques used by a DBMS to process, optimize and execute high-level queries. • The techniques used to split complex queries into multiple simple operations and methods of implementing these low-level operations. • The query optimization techniques are used to chose an efficient execution plan that will minimize the runtime as well as many other types of resources such as number of disk I/O, CPU time and so on.
- 3. Query Processing • Query Processing is a procedure of transforming a high-level query (such as SQL) into a correct and efficient execution plan expressed in low-level language. • When a database system receives a query for update or retrieval of information, it goes through a series of compilation steps, called execution plan. • Query processing goes through various phases: • first phase is called syntax checking phase, the system parses the query and checks that it follows the syntax rules or not. • It then matches the objects in the query syntax with the view tables and columns listed in the system table.
- 4. • In second phase the SQL query is translated in to an algebraic expression using various rules. • So that the process of transforming a high-level SQL query into a relational algebraic form is called Query Decomposition. • The relational algebraic expression now passes to the query optimizer. • In third phase optimization is performed by substituting equivalent expression depends on the factors such that the existence of certain database structures, whether or not a given file is stored, the presence of different indexes & so on. • Query optimization module work in tandem with the join manager module to improve the order in which joins are performed.
- 5. • At this stage the cost model and several other estimation formulas are used to rewrite the query. • The modified query is written to utilize system resources so as to bring the optimal performance. • The query optimizer then generates an action plan also called a execution plan. • This action plans are converted into a query codes that are finally executed by a run time database processor. • The run time database processor estimate the cost of each action plan and chose the optimal one for the execution.
- 6. Query Analyzer • The syntax analyzer takes the query from the users, parses it into tokens and analyses the tokens and their order to make sure they follow the rules of the language grammar. • Is an error is found in the query submitted by the user, it is rejected and an error code together with an explanation of why the query was rejected is return to the user.
- 7. Query Decomposition • In query decomposition the query processing aims are to transfer the high-level query into a relational algebra query and to check whether that query is syntactically and semantically correct. • Thus the query decomposition is start with a high-level query and transform into query graph of low-level operations, which satisfy the query. • The SQL query is decomposed into query blocks (low-level operations), which form the basic unit. • Hence nested queries within a query are identified as separate query blocks. • The query decomposer goes through five stages of processing for decomposition into low-level operation and translation into algebraic expressions.
- 9. Query Analysis • During the query analysis phase, the query is syntactically analyzed using the programming language compiler (parser). • A syntactically legal query is then validated, using the system catalog, to ensure that all data objects (relations and attributes) referred to by the query are defined in the database. • The type specification of the query qualifiers and result is also checked at this stage.
- 10. • Example: SELECT emp_nm FROM EMPLOYEE WHERE emp_desg>100 This query will be rejected because the comparison “>100” is incompatible with the data type of emp_desg which is a variable character string. • At the end of query analysis phase, the high-level query (SQL) is transformed into some internal representation that is more suitable for processing. • This internal representation is typically a kind of query tree.
- 11. • A Query Tree is a tree data structure that corresponds expression. • A Query Tree is also called a relational algebra tree. – Leaf node of the tree, representing the base input relations of the query. – Internal nodes result of applying an operation in the algebra. – Root of the tree representing a result of the query.
- 12. • SELECT (P.proj_no, P.dept_no, E.name, E.add, E.dob) • FROM PROJECT P, DEPARTMENT D, EMPLOYEE E • WHERE P.dept_no = D.d_no AND D.mgr_id = E.emp_id AND P.proj_loc = ‘Mumbai’ ;
- 14. • The three relations PROJECT, DEPARTMENT, EMPLOYEE are represent as a leaf nodes P, D and E, while the relational algebra operations of the represented by internal tree nodes. • Same SQL query can have man different relational algebra expressions and hence many different query trees. • The query parser typically generates a standard initial (canonical) query tree.
- 15. Query Normalization • The primary phase of the normalization is to avoid redundancy. • The normalization phase converts the query into a normalized form that can be more easily manipulated. • In the normalization phase, a set of equivalency rules are applied so that the projection and selection operations included on the query are simplified to avoid redundancy. • The projection operation corresponds to the SELECT clause of SQL query and the selection operation correspond to the predicate found in WHERE clause. • The equivalency transformation rules that are applied.
- 16. Semantic Analyzer • The objective of this phase of query processing is to reduce the number of predicates. • The semantic analyzer rejects the normalized queries that are incorrectly formulated. • A query is incorrectly formulated if components do not contribute to the generation of result. • This happens in case of missing join specification. • A query is contradictory if its predicate cannot satisfy by any tuple in the relation. • The semantic analyzer examine the relational calculus query (SQL) to make sure it contains only data objects that is table, columns, views, indexes that are defined in the database catalog. • It makes sure that each object in the query is referenced correctly according to its data type. • In case of missing join specifications the components do not contribute to the generation of the results, and thus, a query may be incorrect formulated.
- 17. Query Simplifier • The objectives of this phase are to detect redundant qualification, eliminate common sub-expressions and transform sub-graph to semantically equivalent but more easy and efficiently computed form. • Why to simplify? – Commonly integrity constraints, view definitions and access restrictions are introduced into the graph at this stage of analysis so that the query must be simplified as much as possible. – Integrity constraints defines constants which must holds for all state of database, so any query that contradict an integrity constraints must be avoid and can be rejected without accessing the database.
- 18. Query Restructuring • In the final stage of the query decomposition, the query can be restructured to give a more efficient implementation. • Transformation rules are used to convert one relational algebra expression into an equivalent form that is more efficient. • The query can now be regarded as a relational algebra program, consisting of a series of operations on relation.
- 19. Query Optimization • The primary goal of query optimization is of choosing an efficient execution strategy for processing a query. • The query optimizer attempts to minimize the use of certain resources (mainly the number of I/O and CPU time) by selecting a best execution plan (access plan). • A query optimization start during the validation phase by the system to validate the user has appropriate privileges. • Now an action plan is generate to perform the query.
- 20. Block Diagram of Query Optimization
- 21. • Relational algebra query tree generated by the query simplifier module of query decomposer. • Estimation formulas used to determine the cardinality of the intermediate result table. • A cost Model. • Statistical data from the database catalogue. The output of the query optimizer is the execution plan in form of optimized relational algebra query. A query typically has many possible execution strategies, and the process of choosing a suitable one for processing a query is known as Query Optimization.
- 22. The basic issues in Query Optimization • How to use available indexes? • How to use memory to accumulate information and perform immediate steps such as sorting? • How to determine the order in which joins should be performed?
- 23. Objective of query optimization • The term query optimization does not mean giving always an optimal (best) strategy as the execution plan. • It is just a responsibly efficient strategy for execution of the query. • The decomposed query block of SQL is translating into an equivalent extended relational algebra expression and then optimized.
- 24. Techniques for Query Optimization • The first technique is based on Heuristic Rules for ordering the operations in a query execution strategy. • The second technique involves the systematic estimation of the cost of the different execution strategies and choosing the execution plan with the lowest cost.
- 25. • Semantic query optimization is used with the combination with the heuristic query transformation rules. • It uses constraints specified on the database schema such as unique attributes and other more complex constraints, in order to modify one query into another query that is more efficient to execute.
- 26. Heuristic Rules • The heuristic rules are used as an optimization technique to modify the internal representation of query. • Usually, heuristic rules are used in the form of query tree of query graph data structure, to improve its performance. • One of the main heuristic rule is to apply SELECT operation before applying the JOIN or other BINARY operations. • This is because the size of the file resulting from a binary operation such as JOIN is usually a multi- value function of the sizes of the input files.
- 27. Heuristic Rules • The SELECT and PROJECT reduced the size of the file and hence, should be applied before the JOIN or other binary operation. • Heuristic query optimizer transforms the initial (canonical) query tree into final query tree using equivalence transformation rules. • This final query tree is efficient to execute.
- 28. General Transformation Rules • Cascade of σ :- • σ c1 AND c2 AND …AND cn (R) = σ c1 (σ c2 (…(σ cn (R))…)) • Commutativity of σ :- • σ C1 (σ C2 (R)) = σ C2 (σ C1 (R)) Cascade of Л :- • Л List1 (Л List2 (…(Л List n (R))…)) = Л List1 (R) • Commuting σ with Л :- • Л A1,A2,A3…An (σ C (R) ) = σ C (Л A1,A2,A3…An (R))
- 29. General Transformation Rules Commutativity of ⋈ AND x :- R ⋈ c S = S ⋈ c R R x S = S x R • Commuting σ with ⋈ or x :- If all attributes in selection condition c involved only attributes of one of the relation schemas (R). σ c (R ⋈ S) = (σ c (R) ) ⋈ S Alternatively, selection condition c can be written as (c1 AND c2) where condition c1 involves only attributes of R and condition c2 involves only attributes of S then : σ c (R ⋈ S) = (σ c1 (R) ) ⋈ (σ c2 (S) ) • Commuting Л with ⋈ or x :- The projection list L = {A1,A2,..An,B1,B2,…Bm}. A1…An attributes of R and B1…Bm attributes of S. Join condition C involves only attributes in L then : ЛL ( R ⋈ c S ) = ( ЛA1,…An (R) ) ⋈c ( ЛB1,…Bm(S) )
- 30. General Transformation Rules • Commutative of SET Operation :- R ⋃ S = S ⋃ R R ⋂ S = S ⋂ R Minus (R-S) is not commutative. • Associatively of ⋈, x, ⋂, and ⋃ :- If ∅ stands for any one of these operation throughout the expression then : (R ∅ S) ∅ T = R ∅ (S ∅ T) • Commutativity of σ with SET Operation :- If ∅ stands for any one of three operations (⋃,⋂,and-) then : σ c (R ∅ S) = (σ c (R)) ⋃ (σ c (S)) Л c (R ∅ S) = (Л c (R)) ⋃ (Лc (S)) • The Л operation comute with ⋃ :- Л L (R ⋃ S) = (Л L(R)) ⋃ (Л L(S)) • Converting a (σ,x) sequence with ⋃ (σ c (R x S)) = (R ⋈ c S)
- 31. THANK YOU