DB2 SQL Tuning Tips for z/OS Developers

Tony Andrews has more than 23 years’ experience in the development of IBM DB2 relational database applications. Most of this time, he has provided development and consulting services to Fortune 500 companies and government agencies. Tony has written literally thousands of queries and programs during his development years, and he has also served as a DB2 database analyst. For the past 10 years, Tony has been splitting his time between consulting engagements and training. His main focus is to teach today’s developers the ways of RDMS application design, development, and SQL programming— always with a special emphasis on improving performance. Tony’s training, consulting, and speaking engagements are through his employer, Themis, Inc., an onsite and virtual instructor-led, hands-on IT training company recognized internationally. It offers more than 400 IT courses and helps to support International DB2 Users Group North America (IDUG NA) and Europe, Middle East, and Africa (IDUG EMEA), along with many DB2 user groups. Tony is a current IBM champion and regular lecturer at industry conferences and local user groups. You may have seen him present at such events as IDUG NA and EMEA. He is well known for his “Top 25+ Tuning Tips for Developers” presentation. Tony graduated from Ohio State University with a major in business and a minor in mathematical statistics. He currently resides in Dublin, Ohio.

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