Re: Advice in debugging plan selection

[Kathey Marsden]

Kathey Marsden wrote:
One thing I noticed is that usingPredsPushedFromAbove  is false for 
both the passing and failing runs.  I thought it would be true for the 
failing case since we are using the index.

I missed printing out one instance where memoryUsageOK returned false.  
That is in fact the reason the hash join is skipped.  I traced the 
problem to this code  in org.apache.derby.iapi.services.cache.ClassSize 
which sometimes sets refSize to 4, sometimes 5 and sometimes 9.  If it 
is 9 the test fails.


 static
   {
       try
       {
           catalog = (java.util.Hashtable)
             Class.forName( 
"org.apache.derby.iapi.services.cache.ClassSizeCatalog").newInstance();
       }
       catch( Exception e){};

       // Figure out whether this is a 32 or 64 bit machine.
       Runtime runtime = Runtime.getRuntime();
       long memBase = runtime.totalMemory() - runtime.freeMemory();
       Object[] junk = new Object[10000];
       long memUsed = runtime.totalMemory() - runtime.freeMemory() - 
memBase;
       int sz = (int)((memUsed + junk.length/2)/junk.length);
       refSize = ( 4 > sz) ? 4 : sz;
       minObjectSize = 4*refSize;
   }


One thing I don't understand about this code is the line.
       int sz = (int)((memUsed + junk.length/2)/junk.length);

Why is it not just memUsed/junk.length?

Also is it likely a jvm bug that we end up with 9 or are we making wrong 
assumptions about what the jvm will return for totalMemory() and 
freeMemory()?

Thanks

Kathey