IBM Support

MA47178 - LIC-DB-OTHER-PERFM REDUCE USE OF SPARSE MTIS WHEN THERE IS AN
ACCEPTABLE PERM INDEX PER ALWCPYDTA *NO

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 APAR (Authorized Program Analysis Report)

Abstract

LIC-DB-OTHER-PERFM REDUCE USE OF SPARSE MTIS WHEN THERE IS AN
ACCEPTABLE PERM INDEX PER ALWCPYDTA *NO

Error Description

reduce use of sparse MTIs when there is an acceptable perm index
per alwcpydta *NO                                              

Problem Summary

Please see the ERROR DESCRIPTION section.                      

Problem Conclusion

A  number of queries with the allow copy data *NO              
option (via dynamic cursor). These queries are run many times  
and across many connections and cursors can often be left open  
for lengths of time. The customer had adequate permanent        
indexes to service the query ordering and local selection needs
but because the optimizer always cost compares the "perfect"    
sparse MTI against the best permanent index plan, the sparse MTI
option often wins despite the overhead of managing many        
temporary sparse MTIs. The optimizer doesn't really know that  
there would or could be many. This is not usually a problem if  
the query is run relatively few times but when run many times,  
favoring a good permanent index strategy vs sparse MTI is the  
best option. Sparse MTIs have overhead in their creation and    
also in their deletion at cursor close and if there are        
numerous, accumulating sparse MTI this overhead can consume    
system resource to the detriment of applications running on the
IBM i. The solution here is to only cost compare a sparse MTI  
if the servicing permanent index best plan is 'scanning' the    
permanent index for the ordering. If the best index plan is    
servicing both local select and the ordering with at least one  
probe predicate, the optimizer will not consider the sparse MTI.

Temporary Fix

Comments

Circumvention


PTFs Available

R720 MF65153  9123

R730 MF67124  0310

Affected Modules


         
         

Affected Publications

Summary Information

Status............................................CLOSED PER
HIPER...........................................No
Component..................................9400DG3DB
Failing Module..........................RCHMGR
Reported Release...................R720
Duplicate Of..............................




IBM i Support

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Document Information

Modified date:
13 November 2020