1 files changed, 34 insertions, 8 deletions

diff --git a/src/where.c b/src/where.c
index 20823046f..4dd6579e2 100644
--- a/src/where.c
+++ b/src/where.c
@@ -5424,6 +5424,7 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
   LogEst rCost;             /* Cost of a path */
   LogEst nOut;              /* Number of outputs */
   LogEst mxCost = 0;        /* Maximum cost of a set of paths */
+  LogEst mxOut = 0;         /* nOut value for maximum cost path */
   int nTo, nFrom;           /* Number of valid entries in aTo[] and aFrom[] */
   WherePath *aFrom;         /* All nFrom paths at the previous level */
   WherePath *aTo;           /* The nTo best paths at the current level */
@@ -5441,7 +5442,7 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
   ** For joins of 3 or more tables, track the 10 best paths */
   mxChoice = (nLoop<=1) ? 1 : (nLoop==2 ? 5 : 10);
   assert( nLoop<=pWInfo->pTabList->nSrc );
-  WHERETRACE(0x002, ("---- begin solver\n"));
+  WHERETRACE(0x002, ("---- begin solver.  (nRowEst=%d)\n", nRowEst));
 
   /* Allocate and initialize space for aTo and aFrom */
   ii = (sizeof(WherePath)+sizeof(WhereLoop*)*nLoop)*mxChoice*2;
@@ -5529,17 +5530,33 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
         }else{
           revMask = pFrom->revLoop;
         }
-        /* Check to see if pWLoop should be added to the mxChoice best so far */
+        /* Check to see if pWLoop should be added to the set of
+        ** mxChoice best-so-far paths.
+        **
+        ** First look for an existing path among best-so-far paths
+        ** that covers the same set of loops and has the same isOrdered
+        ** setting as the current path candidate.
+        **
+        ** The term "((pTo->isOrdered^isOrdered)&0x80)==0" is equivalent
+        ** to (pTo->isOrdered==(-1))==(isOrdered==(-1))" for the range
+        ** of legal values for isOrdered, -1..64.
+        */
         for(jj=0, pTo=aTo; jj<nTo; jj++, pTo++){
           if( pTo->maskLoop==maskNew
-           && ((pTo->isOrdered^isOrdered)&80)==0
+           && ((pTo->isOrdered^isOrdered)&0x80)==0
           ){
             testcase( jj==nTo-1 );
             break;
           }
         }
         if( jj>=nTo ){
-          if( nTo>=mxChoice && rCost>=mxCost ){
+          /* None of the existing best-so-far paths match the candidate. */
+          if( nTo>=mxChoice
+           && (rCost>mxCost || (rCost==mxCost && nOut>=mxOut))
+          ){
+            /* The current candidate is no better than any of the mxChoice
+            ** paths currently in the best-so-far buffer.  So discard
+            ** this candidate as not viable. */
 #ifdef WHERETRACE_ENABLED /* 0x4 */
             if( sqlite3WhereTrace&0x4 ){
               sqlite3DebugPrintf("Skip   %s cost=%-3d,%3d order=%c\n",
@@ -5549,7 +5566,8 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
 #endif
             continue;
           }
-          /* Add a new Path to the aTo[] set */
+          /* If we reach this points it means that the new candidate path
+          ** needs to be added to the set of best-so-far paths. */
           if( nTo<mxChoice ){
             /* Increase the size of the aTo set by one */
             jj = nTo++;
@@ -5566,7 +5584,11 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
           }
 #endif
         }else{
-          if( pTo->rCost<=rCost ){
+          /* Control reaches here if best-so-far path pTo=aTo[jj] covers the
+          ** same set of loops and has the sam isOrdered setting as the
+          ** candidate path.  Check to see if the candidate should replace
+          ** pTo or if the candidate should be skipped */
+          if( pTo->rCost<rCost || (pTo->rCost==rCost && pTo->nRow<=nOut) ){
 #ifdef WHERETRACE_ENABLED /* 0x4 */
             if( sqlite3WhereTrace&0x4 ){
               sqlite3DebugPrintf(
@@ -5578,11 +5600,13 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
                   pTo->isOrdered>=0 ? pTo->isOrdered+'0' : '?');
             }
 #endif
+            /* Discard the candidate path from further consideration */
             testcase( pTo->rCost==rCost );
             continue;
           }
           testcase( pTo->rCost==rCost+1 );
-          /* A new and better score for a previously created equivalent path */
+          /* Control reaches here if the candidate path is better than the
+          ** pTo path.  Replace pTo with the candidate. */
 #ifdef WHERETRACE_ENABLED /* 0x4 */
           if( sqlite3WhereTrace&0x4 ){
             sqlite3DebugPrintf(
@@ -5606,9 +5630,11 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
         if( nTo>=mxChoice ){
           mxI = 0;
           mxCost = aTo[0].rCost;
+          mxOut = aTo[0].nRow;
           for(jj=1, pTo=&aTo[1]; jj<mxChoice; jj++, pTo++){
-            if( pTo->rCost>mxCost ){
+            if( pTo->rCost>mxCost || (pTo->rCost==mxCost && pTo->nRow>mxOut) ){
               mxCost = pTo->rCost;
+              mxOut = pTo->nRow;
               mxI = jj;
             }
           }