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From abr...@apache.org
Subject svn commit: r615076 - /db/derby/code/branches/10.3/java/engine/org/apache/derby/impl/sql/compile/OptimizerImpl.java
Date Thu, 24 Jan 2008 23:17:31 GMT
Author: abrown
Date: Thu Jan 24 15:17:10 2008
New Revision: 615076

URL: http://svn.apache.org/viewvc?rev=615076&view=rev
Log:
DERBY-3214: Account for loss of precision that can occur when very
large cost estimates are added to, and then subtracted from, relatively
small cumulative estimates during Optimizable "pull" processing.

Merging from trunk to 10.3 branch.  Following commands ran cleanly:

  svn merge -r 603658:603659 https://svn.apache.org/repos/asf/db/derby/code/trunk
  svn merge -r 604512:604513 https://svn.apache.org/repos/asf/db/derby/code/trunk

No other changes were required.

Modified:
    db/derby/code/branches/10.3/java/engine/org/apache/derby/impl/sql/compile/OptimizerImpl.java

Modified: db/derby/code/branches/10.3/java/engine/org/apache/derby/impl/sql/compile/OptimizerImpl.java
URL: http://svn.apache.org/viewvc/db/derby/code/branches/10.3/java/engine/org/apache/derby/impl/sql/compile/OptimizerImpl.java?rev=615076&r1=615075&r2=615076&view=diff
==============================================================================
--- db/derby/code/branches/10.3/java/engine/org/apache/derby/impl/sql/compile/OptimizerImpl.java
(original)
+++ db/derby/code/branches/10.3/java/engine/org/apache/derby/impl/sql/compile/OptimizerImpl.java
Thu Jan 24 15:17:10 2008
@@ -857,10 +857,26 @@
 
 					/*
 					** It's possible for newCost to go negative here due to
-					** loss of precision.
+					** loss of precision--but that should ONLY happen if the
+					** optimizable we just pulled was at position 0.  If we
+					** have a newCost that is <= 0 at any other time, then
+					** it's the result of a different kind of precision loss--
+					** namely, the estimated cost of pullMe was so large that
+					** we lost the precision of the accumulated cost as it
+					** existed prior to pullMe. Then when we subtracted
+					** pullMe's cost out, we ended up setting newCost to zero.
+					** That's an unfortunate side effect of optimizer cost
+					** estimates that grow too large. If that's what happened
+					** here,try to make some sense of things by adding up costs
+					** as they existed prior to pullMe...
 					*/
-					if (newCost < 0.0)
-						newCost = 0.0;
+					if (newCost <= 0.0)
+					{
+						if (joinPosition == 0)
+							newCost = 0.0;
+						else
+							newCost = recoverCostFromProposedJoinOrder(false);
+					}
 				}
 
 				/* If we are choosing a new outer table, then
@@ -934,6 +950,18 @@
 													ap.getCostEstimate().getEstimatedCost();
 						}
 
+						// See discussion above for "newCost"; same applies here.
+						if (prevEstimatedCost <= 0.0)
+						{
+							if (joinPosition == 0)
+								prevEstimatedCost = 0.0;
+							else
+							{
+								prevEstimatedCost =
+									recoverCostFromProposedJoinOrder(true);
+							}
+						}
+
 						currentSortAvoidanceCost.setCost(
 							prevEstimatedCost,
 							prevRowCount,
@@ -1230,6 +1258,45 @@
 			optimizableList.getOptimizable(i).
 				updateBestPlanMap(FromTable.REMOVE_PLAN, this);
 		}
+	}
+
+	/**
+	 * Iterate through all optimizables in the current proposedJoinOrder
+	 * and find the accumulated sum of their estimated costs.  This method
+	 * is used to 'recover' cost estimate sums that have been lost due to
+	 * the addition/subtraction of the cost estimate for the Optimizable
+	 * at position "joinPosition".  Ex. If the total cost for Optimizables
+	 * at positions < joinPosition is 1500, and then the Optimizable at
+	 * joinPosition has an estimated cost of 3.14E40, adding those two
+	 * numbers effectively "loses" the 1500. When we later subtract 3.14E40
+	 * from the total cost estimate (as part of "pull" processing), we'll
+	 * end up with 0 as the result--which is wrong. This method allows us
+	 * to recover the "1500" that we lost in the process of adding and
+	 * subtracting 3.14E40.
+	 */
+	private double recoverCostFromProposedJoinOrder(boolean sortAvoidance)
+		throws StandardException
+	{
+		double recoveredCost = 0.0d;
+		for (int i = 0; i < joinPosition; i++)
+		{
+			if (sortAvoidance)
+			{
+				recoveredCost +=
+					optimizableList.getOptimizable(proposedJoinOrder[i])
+						.getBestSortAvoidancePath().getCostEstimate()
+							.getEstimatedCost();
+			}
+			else
+			{
+				recoveredCost +=
+					optimizableList.getOptimizable(proposedJoinOrder[i])
+						.getBestAccessPath().getCostEstimate()
+							.getEstimatedCost();
+			}
+		}
+
+		return recoveredCost;
 	}
 
 	/*



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