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MA49095 - OSP-OTHER-PERFM INCREASE IN KEY RANGE ESTIMATES

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APAR (Authorized Program Analysis Report)

Abstract

OSP-OTHER-PERFM INCREASE IN KEY RANGE ESTIMATES

Error Description

During optimization of a query, requests for selectivity        
information from indexes can be obtained in the form of key    
range estimate operations. In order to get accurate information
for a given predicate, the statistics manager may need to      
evaluate a significant portion of an index's data. This        
evaluation can take a long time for large indexes as index      
pages are loaded into active memory and then analyzed.          
                                                               
In order to shorten the time required to optimize a query, the  
statistics manager uses several different strategies when faced
with the need to perform a key range estimate.                  
One of the strategies is to cache prior estimates so that      
subsequent requests for the same key range can be returned      
quickly. Each index has a cache associated with it. Estimates  
are stored in this cache until the cache fills or becomes stale
relative to the data in the index or until an IPL occurs. In    
this case, it was determined that key ranges were being        
invalidated too frequently (every couple of days) when in the  
past, they were only invalidated after an IPL.                  
                                                               
ADDITIONAL KEYWORDS: EKR                                        

Problem Summary

During optimization of a query, requests for selectivity        
information from indexes can be obtained in the form of key    
range estimate operations. In order to get accurate information
for a given predicate, the statistics manager may need to      
evaluate a significant portion of an index's data. This        
evaluation can take a long time for large indexes as index pages
are loaded into active memory and then analyzed.                
                                                               
In order to shorten the time required to optimize a query, the  
statistics manager uses several different strategies when faced
with the need to perform a key range estimate.                  
One of the strategies is to cache prior estimates so that      
subsequent requests for the same key range can be returned      
quickly. Each index has a cache associated with it. Estimates  
are stored in this cache until the cache fills or becomes stale
relative to the data in the index or until an IPL occurs. In    
this case, it was determined that key ranges were being        
invalidated too frequently (every couple of days) when in the  
past, they were only invalidated after an IPL.                  
                                                               
ADDITIONAL KEYWORDS: EKR                                        

Problem Conclusion

In some cases, a key range estimate operation may incorrectly  
cause the key range estimate cache to shrink to its minimum size
of 100 entries. When this happens, valid estimates may be purged
from the cache, requiring additional key range processing if    
those estimates are needed again.                              
                                                               
This fix will prevent the incorrect behavior, allowing key range
estimate caches to grow to their intended size. This will      
increase cached estimate re-use and reduce the number of key    
range estimate operations for indexes affected by the incorrect
behavior.                                                      

Temporary Fix

Comments

Circumvention


PTFs Available

R730 MF68793  2132

R740 MF68794  2125

Affected Modules


         
         

Affected Publications

Summary Information

Status............................  CLOSED PER
HIPER.............................  No
Component.........................  9400DG3DB
Failing Module....................  RCHMGR
Reported Release..................  R730
Duplicate Of......................  




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Document Information

Modified date:
24 May 2022