Adbms 37 query optimisation
- 1. Advance Database Management Systems : 37 Query Optimisation Prof Neeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer
- 2. Slide 15- 2 Query optimization • Query optimization: – The process of choosing a suitable execution strategy for processing a query. • Two internal representations of a query: – Query Tree – Query Graph
- 3. Slide 15- 3 Steps in Query Processing
- 4. Slide 15- 4 1. Translating SQL Queries into Relational Algebra (1) • Query block: – The basic unit that can be translated into the algebraic operators and optimized. • A query block contains a single SELECT-FROM- WHERE expression, as well as GROUP BY and HAVING clause if these are part of the block. • Nested queries within a query are identified as separate query blocks. • Aggregate operators in SQL must be included in the extended algebra.
- 5. Slide 15- 5 Translating SQL Queries into Relational Algebra (2) SELECT LNAME, FNAME FROM EMPLOYEE WHERE SALARY > ( SELECT MAX (SALARY) FROM EMPLOYEE WHERE DNO = 5); SELECT MAX (SALARY) FROM EMPLOYEE WHERE DNO = 5 SELECT LNAME, FNAME FROM EMPLOYEE WHERE SALARY > C πLNAME, FNAME (σSALARY>C(EMPLOYEE)) ℱMAX SALARY (σDNO=5 (EMPLOYEE))
- 6. Slide 15- 6 2. Algorithms for External Sorting (1) • External sorting: –Refers to sorting algorithms that are suitable for large files of records stored on disk that do not fit entirely in main memory, such as most database files.
- 7. • Sort-Merge strategy: – Starts by sorting small subfiles (runs) of the main file and then merges the sorted runs, creating larger sorted subfiles that are merged in turn. – Sorting phase: nR = (b/nB) – Merging phase: dM = Min (nB-1, nR); nP = (logdM(nR)) – nR: number of initial runs; b: number of file blocks; – nB: available buffer space; dM: degree of merging; – nP: number of passes.
- 8. Slide 15- 8 Algorithms for External Sorting (2)
- 9. Assignment • Explain the concept of query optimisation