1 files changed, 47 insertions, 6 deletions

diff --git a/src/vdbesort.c b/src/vdbesort.c
index 8b6ecc5e0..8c6bfda9a 100644
--- a/src/vdbesort.c
+++ b/src/vdbesort.c
@@ -973,14 +973,55 @@ int sqlite3VdbeSorterNext(sqlite3 *db, const VdbeCursor *pCsr, int *pbEof){
 
   if( pSorter->aTree ){
     int iPrev = pSorter->aTree[1];/* Index of iterator to advance */
-    int i;                        /* Index of aTree[] to recalculate */
-
     rc = vdbeSorterIterNext(db, &pSorter->aIter[iPrev]);
-    for(i=(pSorter->nTree+iPrev)/2; rc==SQLITE_OK && i>0; i=i/2){
-      rc = vdbeSorterDoCompare(pCsr, i);
-    }
+    if( rc==SQLITE_OK ){
+      int i;                      /* Index of aTree[] to recalculate */
+      VdbeSorterIter *pIter1;     /* First iterator to compare */
+      VdbeSorterIter *pIter2;     /* Second iterator to compare */
+      u8 *pKey2;                  /* To pIter2->aKey, or 0 if record cached */
+
+      /* Find the first two iterators to compare. The one that was just
+      ** advanced (iPrev) and the one next to it in the array.  */
+      pIter1 = &pSorter->aIter[(iPrev & 0xFFFE)];
+      pIter2 = &pSorter->aIter[(iPrev | 0x0001)];
+      pKey2 = pIter2->aKey;
+
+      for(i=(pSorter->nTree+iPrev)/2; i>0; i=i/2){
+        /* Compare pIter1 and pIter2. Store the result in variable iRes. */
+        int iRes;
+        if( pIter1->pFile==0 ){
+          iRes = +1;
+        }else if( pIter2->pFile==0 ){
+          iRes = -1;
+        }else{
+          vdbeSorterCompare(pCsr, 0, 
+              pIter1->aKey, pIter1->nKey, pKey2, pIter2->nKey, &iRes
+          );
+        }
 
-    *pbEof = (pSorter->aIter[pSorter->aTree[1]].pFile==0);
+        /* If pIter1 contained the smaller value, set aTree[i] to its index.
+        ** Then set pIter2 to the next iterator to compare to pIter1. In this
+        ** case there is no cache of pIter2 in pSorter->pUnpacked, so set
+        ** pKey2 to point to the record belonging to pIter2.
+        **
+        ** Alternatively, if pIter2 contains the smaller of the two values,
+        ** set aTree[i] to its index and update pIter1. If vdbeSorterCompare()
+        ** was actually called above, then pSorter->pUnpacked now contains
+        ** a value equivalent to pIter2. So set pKey2 to NULL to prevent
+        ** vdbeSorterCompare() from decoding pIter2 again.  */
+        if( iRes<=0 ){
+          pSorter->aTree[i] = (pIter1 - pSorter->aIter);
+          pIter2 = &pSorter->aIter[ pSorter->aTree[i ^ 0x0001] ];
+          pKey2 = pIter2->aKey;
+        }else{
+          if( pIter1->pFile ) pKey2 = 0;
+          pSorter->aTree[i] = (pIter2 - pSorter->aIter);
+          pIter1 = &pSorter->aIter[ pSorter->aTree[i ^ 0x0001] ];
+        }
+
+      }
+      *pbEof = (pSorter->aIter[pSorter->aTree[1]].pFile==0);
+    }
   }else{
     SorterRecord *pFree = pSorter->pRecord;
     pSorter->pRecord = pFree->pNext;