Query Processing and Optimization

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Unnesting Nested Queries, cont. • Watch out for duplicates! Consider a query <strong>and</strong> its rewritten counterpart. SELECT AVG(salary) FROM emp WHERE manager IN (SELECT manager FROM techdept) • Unnested version, with problems: (what’s the problem?) SELECT AVG(salary) FROM emp, techdept WHERE emp.manager = techdept.manager • This query may yield wrong results! A solution: SELECT DISTINCT (manager) INTO temp FROM techdept SELECT AVG(salary) FROM emp, temp WHERE emp. manager = temp. manager 52
Summary • <strong>Query</strong> processing & optimization is the heart of a relational DBMS. • Heuristic optimization is more efficient to generate, but may not yield the optimal query evaluation plan. • Cost-based optimization relies on statistics gathered on the relations (the default in most DBMSs). • Until query optimization is perfected, query tuning will be a fact of life. 53
Page 1 and 2: Query Processing + Optimization: Ou
Page 3 and 4: Architectural Context DDL Statement
Page 5 and 6: Overview of Query Processing 1. Par
Page 7 and 8: • Linear search Selection Strateg
Page 9 and 10: Joins • Join Strategies • Neste
Page 11 and 12: Nested Loop • Basically, for each
Page 13 and 14: Index-based Join • Requires (at l
Page 15 and 16: Sort-Merge Join • Sort each relat
Page 17 and 18: External or Disk-based Sorting, Con
Page 19 and 20: Example of Sort-Merge Join • Cost
Page 21 and 22: Partitioning Phase • Partitioning
Page 23 and 24: Hash Join Cost • Cost Total = Cos
Page 25 and 26: Cost and Applicability of Join Stra
Page 27 and 28: Query Optimization • Aim: Transfo
Page 29 and 30: Apply Selections Early π Name σ F
Page 31 and 32: Form Joins π Name n CU.CustomerID
Page 33 and 34: Some Transformation Rules • Casca
Page 35 and 36: Heuristic Query Optimization Summar
Page 37 and 38: • Per relation • Tuple size Rel
Page 39 and 40: • Per Index • Base relation Rel
Page 41 and 42: • Statistics Operation 1: σ foll
Page 43 and 44: • Statistics Operation 3: σ foll
Page 45 and 46: Comparison • Heuristic query opti
Page 47 and 48: Query Tuning • Query optimization
Page 49 and 50: Join on Clustering Index, and Integ
Page 51: Unnesting Nested Queries • Uncorr

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