DB2 SQL Tuning Tips for z/OS Developers

CHAPTER 1 SQL Optimization Top 100+ 1
1. Take Out Any/All Scalar Functions Coded on Columns in Predicates 2
2. Take Out Any/All Mathematics Coded on Columns in Predicates 3
3. Code Only the Columns Needed in the Select Portion of the SQL Statement 4
4. Stay Away from Distinct if Possible 4
5. Try Rewriting an In Subquery as an Exists Subquery 5
6. Always Make Sure Host Variables Are Defined to Match the Columns Datatype 6
7. Because Or Logic Can Be Problematic to the Optimizer, Try a Different Rewrite 6
8. Make Sure the Data Distribution and Other Statistics Are Good and Current in the Tables Being Processed 8
9. Use UNION ALL in Place of UNION Where Possible 9
10. Consider Hardcoding Versus Using a Host Variable 9
11. Minimize DB2’s SQL Requests 11
12. Try Rewriting Range Predicates as Between Predicates 12
13. Consider Using Global Temporary Tables 13
14. Give Prominence to Stage 1 over Stage 2 Predicates 14
15. Remember That the Order of (Some) Predicates Does Matter 15
16. Streamline Multiple Subqueries 16
17. Index Correlated Subqueries 17
18. Get to Know the DB2 Explain Tool 17
19. Use Tools for Monitoring 18
20. Employ Commit and Restart Strategies 19
21. Implement Good Index Design 19
22. Avoid Discrepancies with Non-Column Expressions 20
23. Begin with All Filtering Logic Outside Application Code 21
24. Ensure That Subquery Predicates Involving Min and Max Have the Possibility of Nulls Being Returned Handled 21
25. Always Code For Fetch Only or For Read Only with Cursor Processing When a Query Is Only Selecting Data 22
26. Avoid Selecting a Row from a Table to Help Decide Whether the Logic in the Code Should Execute an
Update or an Insert 23
27. Avoid Selecting a Row from a Table in Order to Get Values for an Update 23
28. Make Use of Dynamic SQL Statement Caching 23
29. Avoid Using Select * 24
30. Watch Out for Nullable Columns or Times When SQL Statements Could Have Nulls Returned from the Database Manager 25
31. Minimize the Number of Times Open and Close Cursors Are Executed 25
32. Avoid Not Logic in SQL 26
33. Use Correlation IDs for Better Readability 26
34. Keep Table and Index Files Healthy and Organized 27
35. Take Advantage of Update Where Current of Cursor and Delete Where Current of Cursor 27
36. When Using Cursors, Use ROWSET Positioning and Fetching Using Multiple-Row Fetch, Multiple-Row Update, and Multiple-Row Insert 28
37. Know the Locking Isolation Levels 28
38. Know Null Processing 30
39. Always Program with Performance in Mind 31
40. Let SQL Do the Work 32
41. Code with Lock Table 32
42. Consider OLTP Front-End Processing 33
43. Consider Using Dynamic Scrollable Cursors 34
44. Take Advantage of Materialized Query Tables to Improve Response Time (Dynamic SQL Only) 35
45. Insert with Select 37
46. Take Advantage of Multiple-Row Fetch 38
47. Take Advantage of Multiple-Row Insert 39
48. Take Advantage of Multiple-Row Update 40
49. Take Advantage of Multiple-Row Delete 42
50. Try Scalar Fullselects Within the Select Clause 42
51. Take Advantage of REOPT ONCE and REOPT AUTO in Dynamic SQL and REOPT VARS and REOPT ALWAYS in Static SQL 43
52. Identify Times for Volatile Tables 44
53. Use the ON COMMIT DROP Enhancement 45
54. Use Multiple Distincts 45
55. Take Advantage of Backward Index Scanning 46
56. Watch Out for the Like Statement 46
57. Set Your Clustering Index Correctly 47
58. Use Group By Expressions if Needed 48
59. Watch Out for Tablespace Scans 48
60. Do Not Ask for What You Already Know 49
61. Watch the Order of Tables in a Query 49
62. Use Left Outer Joins Over Right Outer Joins 51
63. Check for Non-Existence 51
64. Use Stored Procedures 52
65. Do Not Select a Column in Order to Sort on It 53
66. Always Limit the Result Set if Possible 53
67. Take Advantage of DB2 V8 Enhanced DISCARD Capabilities When It Comes to Mass Deletes 54
68. Take Advantage of the DB2 LOAD Utility for Mass Inserts 54
69. Watch Out for Materialization of Views, Nested Table Expressions, and Common Table Expressions 55
70. Consider Compressing Data 56
71. Consider Parallelism 57
72. Keep the STDDEV, STDDEV_SAMP, VAR, and VAR_SAMP Functions Apart from Other Functions 58
73. Consider Direct Row Access Using ROWID Datatype (V8) or RID Function (V9) 58
74. Test Your Queries with Realistic Statistics and a Level of Data to Reflect Performance Issues 60
75. Specify the Leading Index Columns in WHERE Clauses 61
76. Use WHERE Instead of HAVING for Filtering Whenever Possible 62
77. Keep in Mind Index Only Processing Whenever Possible 62
78. Index on Expression in DB2 V9 63
79. Consider the DB2 V9 Truncate Statement 64
80. Use DB2 V9 Fetch First and Order by Within Subqueries 65
81. Take Advantage of DB2 V9 Optimistic Locking 65
82. Use the DB2 V9 MERGE Statement 66
83. Understand the DB2 NOFOR Precompile Option 68
84. Consider Select Into Using Order By 69
85. Code Boolean Term Predicates Whenever Possible 69
86. Try Transitive Closure Coding 70
87. Avoid Sorts with Order By 71
88. Use Joins Instead of Subqueries Whenever Possible 71
89. Watch Out for Case Logic 71
90. Take Advantage of Functions in the Order By Clause 72
91. Know Your Version of DB2 72
92. Understand Date Arithmetic 73
93. Know Your High-Volume Insert Choices 73
94. Know About Skip Locked Data (V9) for Lock Avoidance. . . . . .75
95. Sort Your Input Streams 75
96. If You Need True Uniqueness, Try the V8 Generate_Unique Function 76
97. Know the New Options for Declared Temporary Tables 76
98. Watch Out When Executing Get Diagnostics 77
99. Order Your In List Appropriately 77
100. Update and Delete with Select (V9) 77
101. Execute SQL Statements Only if Necessary 78
102. Take Advantage of In-Memory Tables 78
103. Stay Away from Catchall SQL Statements 79
104. Avoid Unnecessary Sorting 79
105. Understand Expressions and Column Functions 79
106. Watch Out When Combining Predicates 80
107. Add Redundant Predicates to Search Queries 80
108. Take Advantage of Improved Dynamic Caching (V10) 81
109. Try Currently Committed for Lock Avoidance (V10) 82
110. Try System Temporal Tables for Historical Data (V10) 83
111. Try Business Temporal Tables for Historical Data (V10) 85
112. Know Your Ranking Functions (V10) 86
113. Take Advantage of Extended Indicators (V10) 87
114. Get Greater Timestamp Precision (V10) 88
115. Try Index Includes (V10) 89
116. Use With Return to Client (V10) 89
CHAPTER 2 DB2 SQL Hints 91
1. Try the Optimize for 1 Row Statement at the End of the SQL Statement 91
2. Add the A.PKEY = A.PKEY Predicate to the SQL Query, Where PKEY Equals the Primary Key Column of the Table 92
3. Disqualify an Index Choice 93
4. Change the Order of Table Processing 95
5. Use Distributed Dynamic SQL 96
CHAPTER 3 SQL Standards and Guidelines 99
For COBOL Developers 99
For All SQL Developers 102
CHAPTER 4 SQL Program Walkthroughs 107
CHAPTER 5 Existence Checking 111
Example 1 111
Example 2 113
CHAPTER 6 Runstats 115
CHAPTER 7 Initial Steps in Tuning a Query 117
APPENDIX A Predicate Rewrite Examples 121
Predicate Rewrites: Transitive Closure 122
APPENDIX B DB2 SQL Terminology 125
Index 131