"DB2 Tuning Dynamic SQL

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October 14, 2013
DB2 for z/OS: Tuning
Dynamic SQL
Randy Ebersole
IBM DB2 for z/OS Lab Services
November 20, 2013
© 2013 IBM Corporation
© 2013 IBM Corporation
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© 2013 IBM Corporation
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Agenda
▪ Dynamic SQL Review
▪ Dynamic SQL Performance Considerations
▪ Identifying Dynamic SQL for Tuning
▪ Tuning Tools
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But First.....
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TCP/IP
TCP/IP
TCP/IP
Clients
App
Server
DS
Driver
DB2
z/OS
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The Transaction Manager, Connections, Threads
• DDF is DB2’s transaction manager for distributed database
connections
• With DRDA, connections can come from literally anywhere
• DDF has very mature thread management strategies
• DDF runs as an additional address space in the DB2 subsystem
– The address space name is xxxxDIST
– DDF uses SRBs• instead
of TCBs,
which reduces CPU time.
Click to
add text
MVS enclaves are used in exchanging
• Enclaves are used across address spaces for
communication
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Dynamic SQL
▪ Dynamic SQL is becoming more prevalent
 Programming models
 Vended applications
▪ Dynamic SQL is evaluated for cost-based
optimization at execution
 “Full Prepare” - Code path for executing dynamic SQL
includes DB2 Optimizer access path decision
 “Short Prepare” – Dynamic SQL statement matches
statement found in global Dynamic Statement Cache
– Access path reused
– DSNZPARM CACHEDYN=YES
 CACHEDYN=NO only appropriate if no dynamic SQL at all
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Dynamic SQL, continued
▪ “Prepare Avoided” – Prepared dynamic SQL
statement string saved in local thread storage
 If statement reused, no need to call Dynamic Statement
Cache (DSC)
– Locally cached statement reused unless new PREPARE for
same statement
 DSNZPARM MAXKEEPD limits number of statements
cached in local thread storage
– Virtual storage considerations, especially prior to DB2 10
 BIND option KEEPDYNAMIC YES keeps prepared
dynamic SQL statement in local thread storage across
COMMIT
– Avoid cost of subsequent PREPARE after each COMMIT
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Dynamic SQL Performance Considerations
▪ Full prepare can cost many times that of
equivalent static SQL statements
 E.g: in one case of typical transaction workload, 3x CPU
when code change made short prepare impossible
▪ Short prepare (using dynamic statement cache, or
DSC) will still be more expensive than equivalent
static SQL statement
 Short prepare requires character by character match of
statement in DSC
▪ Local statement cache can approach, or in some
cases exceed, performance of equivalent static
SQL
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Dynamic Statement Cache
▪ On execution of dynamic SQL, DB2 searches DSC for
matching SQL statement
 If no match, new statement added to DSC
 Older statements evicted from DSC
▪ Increasing hit ratio in DSC improves performance of
dynamic SQL
 No match in DSC if literals in predicates change
▪ Use parameter markers, “?”, for literal values that change
frequently
 Parameter markers allow match in DSC
 Do NOT use parameter markers for ALL predicates;
– Better for predicates for which values change
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Concentrate Literals
▪ Some applications do not use parameter markers
 3rd party SW, Code generators, programmers just won’t
▪ DB2 10 for z/OS provides CONCENTRATE
LITERALS
 Uses “&” instead of “?”
▪ BIND option and statement option





Allows DB2 to replace all literals with “&”
May improve match in DSC
May reduce efficiency for literals that do not change
Better for transactional type work
You may not have access to statement level !
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Dynamic SQL Performance Hierarchy
Not to scale
BEST
▪ SQL statement string in local statement cache
▪ SQL statements with intelligent use of parameter
markers
 E.g. if predicate on “LastName” use ‘?’
 If predicate for status code with 10 values, use literal
– Only 10 statements in DSC
▪ SQL statements with Concentrate Literals
▪ Packages with Concentrate Literals
▪ SQL statements with frequently changing literals
▪ All full prepares (e.g. CACHEDYN = NO)
WORST
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Dynamic SQL: Costs of Full and Short Prepare
SQL Complexity
SQL Execution Cost
b
Full Prepare
a
Short Prepare
Cost
Total cost of SQL = prepare cost + execution cost
In ‘a’, short + execution far less than full + execution, and ratio of full to execution is very high
In ‘b’, short + execution less than full + execution, but ratio of full compared to execution is lower, and
potential better access path with full prepare may be worthwhile
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Tuning Dynamic SQL - Challenges
▪ Even with excellent hit ratio in DSC, there may be bad
access paths that are reused
 ‘Excellent’ >= 95%
▪ How to identify problem dynamic SQL statements?
 Infrequently used dynamic SQL will not stay in DSC long
▪ Once identified, how to tune?
▪ Generally RUNSTATS on table or index will evict
statements from DSC that depend on that table or index
 Subsequent execution requires full prepare
▪ Plan Management does not apply to dynamic SQL
 But Optimizer Hints can
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Identifying Dynamic SQL for Tuning
▪ EXPLAIN Dynamic Statement Cache
▪ Sort resulting statements by total CPU consumption
▪ For top consumers, evaluate Getpage Ratio
 Ratio of Getpages to Rows Returned
 High Getpage Ratio suggests tuning opportunity
▪ Use tools to find tuning problems
 Use acquired skills, intuition, etc.
 Few DB2 professionals make best use of tools
▪ Test performance of hypothetical new path
▪ Evaluate results
▪ Repeat
© 2013 IBM Corporation
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EXPLAIN Dynamic Statement Cache
▪ Sample procedure
 Create tables (DSNTESC in SDSNSAMP)
–
–
–
–
DSN_STATEMENT_CACHE_TABLE for execution stats data
DSN_FUNCTION_TABLE
DSN_STATEMENT_TABLE
PLAN_TABLE
 Start collection of DSC performance statistics
– -START TRACE(P) CLASS(30) IFCID(316 317, 318)
 Use EXPLAIN STMTCACHE ALL to extract SQL
statements from DSC and dump statistics to
DSN_STATEMENT_CACHE_TABLE
© 2013 IBM Corporation
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Changes in Dynamic SQL Performance
▪ How do you determine if the performance of your dynamic
SQL has changed?
 Monitors?
– If so, which one?
 User calls to Help Desk?
– “I ran the VERY SAME query on Friday, but today it still has not
responded?”
 Except on Friday ‘STATE’ = ‘AK’ and today ‘STATE’ = ‘CA’
 Other?
▪ If you do not have historical information about your SQL it
will be difficult to tune effectively
 Note: just because it has always run this way does NOT mean it is
good!
▪ Monitor tuned statements as well as largest consumers
 If cannot validate value of tuning, nobody will report the benefit
© 2013 IBM Corporation
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Tuning the Problem SQL Statements
▪ So, now you have found some problem SQL
statements….
▪ What do you do?
▪ Tuning dynamic SQL statements is different from
tuning static SQL statements, because…
 NOT !
 Same issues, same approaches, same tools
– With a couple exceptions
© 2013 IBM Corporation
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SQL Tuning Tools
▪ Different tools have different approaches and offer different
benefits
 But same tools can be used for static or dynamic SQL
▪ IBM tools include:
 Query Monitor
– Take advantage of IFCID 316, 317, 318 data
 InfoSphere Optim Query Workload Tuner
– Rather than analyzing a single statement, analyze a set of statements
that make up a workload
 Data Studio
– Access Path Graphs, RUNSTATS Advisor, Query Capture Environment
 DB2 Connect Unlimited Advanced Edition for System z
– Includes: InfoSphere Optim pureQuery for LUW
 Capture dynamic SQL, convert to static SQL, increase security
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Data Studio
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Query Hints
▪ Tell the Optimizer what you want it to do…
 As a last ditch effort, assisted by level 2, use access
path hints in DB2
 Determine if the 80/20 rule applies and which access
path is acceptable for the majority of the time
 Determine if BIND(QUERY) is reasonable
 Try Selectivity Override (DB2 11)
– DSN_PREDICATE_SELECTIVITY
 Skew your own stats
▪ Put access path hint in DSN_USERQUERY, BIND
query, then flush user query table
© 2013 IBM Corporation
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What if Tuning Efforts Do Not Work?
▪ If you suspect Optimizer problem
 Gather data for Level 2
– https://www.ibm.com/support/docview.wss?uid=swg21206998
– http://www.ibm.com/developerworks/data/library/techarticle/dm1012capturequery/index.html
– SYSPROC.ADMIN_INFO_SQL supports V8 -> V11 (Required)
 Data Studio V4.1 incorporates this procedure into a GUI
(Best Practice)
–
–
–
–
–
http://www.ibm.com/developerworks/downloads/im/data/
No charge product (replacement for OSC and Visual Explain)
Incorporates Statistics Advisor
Query Environment Capture used to collect doc
FTP doc directly to DB2 Level 2 in the tool
© 2013 IBM Corporation
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High Performance DBATs
•
Re-introducing RELEASE(DEALLOCATE) in distributed package
– Could not break in to do system maintenance - Utility or DDL
– V6 PQ63185 to disable RELEASE(DEALLOACTE) on DRDA DBATs
•
High Performance DBATs reduce CPU consumption by
– RELEASE(DEALLOCATE) to avoid repeated package allocation/deallocation
– Avoids processing to go inactive and then back to active
– Bigger CPU reduction for short transactions that commit often
•
Using High Performance DBATs
– Stay active if there is at least one RELEASE(DEALLOCATE) package exists
• Click to add text
– Connections will turn inactive after 200 commits to free up resources
– Normal idle thread time-out detection will be applied to these DBATs
– Good match with JCC packages (handful packages)
– No benefit and not support for ACTIVE threads (CMSTATS=ACTIVE)
– No benefit for KEEPDYNAMIC YES users
© 2013 IBM Corporation
Enabling High Performance DBATs


Two steps to enable High Performance DBAT
1. REBIND with RELEASE(DEALLOCATE)


Default BIND option in JCC driver will be RELEASE (DEALLOCATE) for JCC
Driver matching with DB2 10 (JCC 9.7 FP3a)
2. Then command –MODIFY DDF PKGREL (BNDOPT)

• DISPLAY DDF shows the option currently used

To disable - MODIFY DDF PKGREL (COMMIT) to overlay BNDOPT option

To monitor - Statistics GLOBAL DDF Activity section
• DDF
Click
to add text
GLOBAL
ACTIVITY
CUR ACTIVE DBATS-BND DEALLC
HWM ACTIVE DBATS-BND DEALLC
QUANTITY
5.3
10.00
© 2013 IBM Corporation
Summary
▪
▪
▪
▪
Dynamic SQL use is growing and growing
Many options in place to manage
Take advantage of all of the functionality
It's the “same” as static SQL
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Questions?
THANK YOU !
[email protected]
© 2013 IBM Corporation
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