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DB2 SQL Tuning Tips for Z/OS Developers [Paperback]

Tony Andrews

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Table of Contents

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

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