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SQL Coding Best Practices for Developers

Platform: DB2 for Linux, UNIX, and Windows. SQL Coding Best Practices for Developers. Phil Gunning Principal Consultant, Gunning Technology Solutions, LLC Session: G2 May 23, 2005 12:30 – 1:40. Outline . Best Practices Classes of Predicates Index SARGable Range Delimiting

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SQL Coding Best Practices for Developers

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  1. Platform: DB2 for Linux, UNIX, and Windows SQL Coding Best Practicesfor Developers • Phil Gunning • Principal Consultant, • Gunning Technology Solutions, LLC • Session: G2 • May 23, 2005 • 12:30 – 1:40

  2. Outline • Best Practices • Classes of Predicates • Index SARGable • Range Delimiting • Data SARGable • Predicate Best Practices • Local, Order By, Join Predicates • Constraining Results • Filter Factors • Defaults/Formulas • DB2 Catalog Queries/Explain • Sources of Filter Factor information

  3. Outline • Index Design • Local, Order By, Join predicates • Include Columns • Uniqueness • DB2 Visual Explain/db2exfmt/Design Advisor • Monitor and Evaluate • Summary

  4. Best Practices 1. Use Range Delimiting and Index SARGable Predicates wherever possible 2. Understand DB2 predicate rules 3. Specify most restrictive predicates first 4. Select only columns that are needed 5. Adhere to proper index design techniques 6. Understand inputs to the Optimizer 7. Developers and DBAs collaborate to design proper indexes 8. Evaluate all SQL using Visual Explain/db2exfmt 9. Use Design Advisor to tune SQL/SQL Workloads 10. Consistently monitor and review application performance

  5. Application Relational Data Services Residual predicates COST Data Management Services Data SARGable predicates Range Delimiting Index SARGable Index Manager Data

  6. Classes of Predicates • Range Delimiting • Index SARGable • Predicates that can use an index for a search argument • Resolved by Index Manager • Data SARGable

  7. Predicates • Index SARGable • Are not used to bracket an index scan • Can be evaluated from the index if one is chosen • Evaluated by the Index Manager

  8. Index SARGable Example • Col A = 9 and Col C = 4 • Col A can be used as a range delimiting (start-stop) predicate • Col C can be used as an Index SARGable predicate, it cannot be used as a range delimiting since there is no predicate on Col B • Starting with columns in the index, from left to right, the first inequality predicate stops the column matching

  9. Predicates • Range Delimiting • Used to bracket an index scan • Uses start and stop predicates • Evaluated by the Index Manager

  10. Range Delimiting Example • Col A = 3 and Col B = 6 and Col C = 8 • In this case the equality predicates on all the columns of the index can be applied as start-stop keys and they are all range delimiting

  11. Predicates • Data SARGable • Cannot be evaluated by the Index Manager • Evaluated by Data Management Services • Require the access of individual rows from the base table

  12. Data SARGable Example • Col A = 3 and Col B <= 6 and Col D = 9 • Col A is used as a start-stop predicate, Col B is used as a stop predicate, and Col D which is not present in the index is applied as a Data SARGable predicate during the FETCH from the table

  13. Predicates • Residual Predicates • Cannot be evaluated by the Index Manager • Cannot be evaluated by Data Management Services • Require IO beyond accessing the base table • Predicates such as those using quantified sub-queries (ANY, ALL, SOME, or IN), LONG VARCHAR, or LOB data which is stored separately from the table • Are evaluated by Relational Data Services and are the most expensive type of predicates

  14. Residual Predicate Example • Col B = 4 and UDF with external action(Col D) • In this case the leading Col A does not have a predicate • Col B can only be used as an Index SARGable predicate (where the whole index is scanned) • Col D involves a user defined function which will be applied as a residual predicate

  15. RULE#1 • Use range delimiting and Index SARGable predicates whenever possible

  16. Index Review • An index is a data structure that contains column values and a pointer to the table data • Primary key – Unique Index • If a primary key is defined, DB2 automatically creates a unique index to enforce the PK constraint • Secondary Index • Created to support access to frequently referenced columns • Indexes provide efficient access (in terms of CPU and IO) to columns found in the table • Just like an index entry in a book, an index in a database enables rapid lookup of associated table entries

  17. Index Characteristics • Index entries are usually much smaller (subset) of all table columns • Can fit more index entries on a page • Allows for more efficient use of buffer pool • Separate index buffer pool • Enables often used index pages to remain in the buffer pool longer • More logical IO than physical IO

  18. A Word About Index Structures • B+ -tree used to store index entries • Provides for a tree structure that is balanced to a constant depth from the root to the leaf blocks along every branch • Usually more efficient (less costly) than a table scan

  19. Select deptnumb, deptname from db2admin.org Where deptnumb < 20

  20. Select deptnumb, deptname from db2admin.org Where deptnumb =20 and deptname like 'b%' or division = 'midwest' and manager = 88 or location like 'bo%'

  21. Table Scan Rules of Thumb • If > 20-25% of the rows will be read, good likelihood of table scan • If 0.5 – 20% of the rows are read, likely index access but this can vary depending on numerous factors • Exact formulas used are complex and not very useful for practical purposes

  22. Rule #2 • Understand and apply DB2 predicate rules

  23. WITH DEPT_MGR AS ( SELECT DEPTNO, DEPTNAME, EMPNO, LASTNAME, FIRSTNME, PHONENO FROM DEPARTMENT D, EMPLOYEE E WHERE D.MGRNO=E.EMPNO AND E.JOB='MANAGER' ), DEPT_NO_MGR AS ( SELECT DEPTNO, DEPTNAME, MGRNO AS EMPNO FROM DEPARTMENT EXCEPT ALL SELECT DEPTNO, DEPTNAME, EMPNO FROM DEPT_MGR ), MGR_NO_DEPT (DEPTNO, EMPNO, LASTNAME, FIRSTNME, PHONENO) AS ( SELECT WORKDEPT, EMPNO, LASTNAME, FIRSTNME, PHONENO FROM EMPLOYEE WHERE JOB='MANAGER' EXCEPT ALL SELECT DEPTNO,EMPNO, LASTNAME, FIRSTNME, PHONENO FROM DEPT_MGR ) SELECT DEPTNO, DEPTNAME, EMPNO, LASTNAME, FIRSTNME, PHONENO FROM DEPT_MGR UNION ALL SELECT DEPTNO, DEPTNAME, EMPNO, CAST(NULL AS VARCHAR(15)) AS LASTNAME, CAST(NULL AS VARCHAR(12)) AS FIRSTNME, CAST(NULL AS CHAR(4)) AS PHONENO FROM DEPT_NO_MGR UNION ALL SELECT DEPTNO, CAST(NULL AS VARCHAR(29)) AS DEPTNAME, EMPNO, LASTNAME, FIRSTNME, PHONENO FROM MGR_NO_DEPT ORDER BY 4

  24. Created Two Indexes CREATE INDEX "DB2ADMIN"."AABB" ON "DB2ADMIN"."DEPARTMENT" ("DEPTNO" ASC, "DEPTNAME" ASC, "MGRNO" ASC) PCTFREE 10 CLUSTER MINPCTUSED 10 ALLOW REVERSE SCANS; CREATE INDEX "DB2ADMIN"."CCDD" ON "DB2ADMIN"."EMPLOYEE" ("EMPNO" ASC, "FIRSTNME" ASC, "MIDINIT" ASC, "LASTNAME" ASC, "WORKDEPT" ASC, "PHONENO" ASC) PCTFREE 10 MINPCTUSED 10 ALLOW REVERSE SCANS;

  25. Read one node at each intermediate level Index on DEPTNO, DEPTNAME, MGRNO Read leaf nodes by following sibling pointers until no matching entry is found Index Scan DEPTNAME DEPTNO MGRNO

  26. Selectivity Catalog Queries • SELECT INDNAME, NPAGES, CARD, FIRSTKEYCARD AS FIRSTK, FIRST2KEYCARD AS F2KEY, FIRST3KEYCARD AS F3KEY, FIRST4KEYCARD AS F4KEY, FULLKEYCARD AS FULLKEY, NLEAF, NLEVELS AS NLEV, CLUSTERRATIO AS CR, CLUSTERFACTOR AS CF, UNIQUERULE AS U, T.COLCOUNT AS TBCOL, I.COLCOUNT AS IXCOL FROM SYSCAT.TABLES T, SYSCAT.INDEXES I WHERE T.TABSCHEMA = I.TABSCHEMA AND T.TABSCHEMA = ‘PGUNNING' AND T.TABNAME = I.TABNAME AND CARD >20000 ORDER BY CARD DESC, 1;

  27. XBOOKING1 Selectivity = Number of Distinct Values / CARD 1229/389151 = .003 Meets our rule for selectivity < .10 XBOOKING2 Selectivity = Number of Distinct Values / CARD 111217/389151 = .285 Does not meets our rule for selectivity < .10

  28. Data Specification • Specify the most restrictive predicates first • Select only those columns needed • Use business sense when developing reports for end users • They should not be so voluminous that the average end user will not be able to use them anyway • Haven’t we all seen these monster reports that consume lots of CPU and IO and never get looked at?

  29. Fast Retrieval • OPTIMIZE FOR N ROWS CLAUSE • Can guide the optimizer to use an access path to quickly return N Rows • Also effects the size of the number of rows blocked in the communications buffer • Useful when the number of rows you want is significantly less than total number of rows that could be returned • Can slow performance if most of the rows are going to be processed

  30. Fetch First • FETCH FIRST N ROWS ONLY CLAUSE • Used to restrict fetching to only N rows regardless of number of rows that there may have been in the result set if not specified • FOR FETCH ONLY CLAUSE • Use when no updates are planned • Query can take advantage of row blocking • Only S locks taken on rows retrieved • Improved concurrency

  31. Rule #3 & 4 • Specify most restrictive predicates first • Select only those columns needed

  32. Selectivity • Selectivity of an index column indicates the number of rows that will satisfy the predicate condition • Formula: • Selectivity = number of distinct values / number of rows in the table • Selectivity of predicates should be < .10, that is will return less than 10% of the table to the requesting application or to the intermediate result set if more than a two-way join

  33. Index Design • Indexes should be created on local, order by and join predicates • Frequently access columns with good selectivity • Number of Indexes • Determined by business rules • OLTP • 3-5 indexes • Fewer indexes offer fewer choices to the optimizer • DW • 5 or more

  34. Rule#5 • Adhere to proper index design techniques

  35. DB2 Optimizer • What inputs does the Optimizer consider/analyze during statement optimization? • Important to know as some of these inputs can cause suboptimal access paths if not current • RUNSTATS not current • Buffer pool changes • Configuration parameter changes

  36. REOPT Bind Option • Can be used to enable query reoptimization for dynamic and static SQL that have host variables, parameter markers or special registers • Can set the REOPT option to one of three values • None – No reoptimzation will take place, the default behavior • Once – the access plan will use real values the first time and the plan will be cached in the package cache • Always – the access path will always be compiled and reoptimized using the values of the parameter markers, host variables, or special registers known at each execution time

  37. Lock Wait Mode • Application can specify individual lock wait mode strategy • Take one of the following actions when it cannot obtain a lock: • Return and SQLCODE or SQLSTATE • Wait indefinitely for a lock • Wait a specified amount of time for a lock • Use value of locktimeout DB CFG parameter • SET CURRENT LOCK TIMEOUT statement • Specifies number of seconds to wait for a lock • Applies to row, table, index key, and MDC block locks

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