Index: src/sqliteInt.h
==================================================================
--- src/sqliteInt.h
+++ src/sqliteInt.h
@@ -3569,12 +3569,11 @@
   int iEphCsr;            /* Partition buffer or Peer buffer */
   int regAccum;
   int regResult;
   int csrApp;             /* Function cursor (used by min/max) */
   int regApp;             /* Function register (also used by min/max) */
-  int regPart;            /* First in a set of registers holding PARTITION BY
-                          ** and ORDER BY values for the window */
+  int regPart;            /* Array of registers for PARTITION BY values */
   Expr *pOwner;           /* Expression object this window is attached to */
   int nBufferCol;         /* Number of columns in buffer table */
   int iArgCol;            /* Offset of first argument for this function */
   int regFirst;
 };

Index: src/window.c
==================================================================
--- src/window.c
+++ src/window.c
@@ -1116,16 +1116,18 @@
 ** and initialize registers and cursors used by sqlite3WindowCodeStep().
 */
 void sqlite3WindowCodeInit(Parse *pParse, Window *pMWin){
   Window *pWin;
   Vdbe *v = sqlite3GetVdbe(pParse);
-  int nPart = (pMWin->pPartition ? pMWin->pPartition->nExpr : 0);
-  nPart += (pMWin->pOrderBy ? pMWin->pOrderBy->nExpr : 0);
-  if( nPart ){
+
+  /* Allocate registers to use for PARTITION BY values, if any. Initialize
+  ** said registers to NULL.  */
+  if( pMWin->pPartition ){
+    int nExpr = pMWin->pPartition->nExpr;
     pMWin->regPart = pParse->nMem+1;
-    pParse->nMem += nPart;
-    sqlite3VdbeAddOp3(v, OP_Null, 0, pMWin->regPart, pMWin->regPart+nPart-1);
+    pParse->nMem += nExpr;
+    sqlite3VdbeAddOp3(v, OP_Null, 0, pMWin->regPart, pMWin->regPart+nExpr-1);
   }
 
   pMWin->regFirst = ++pParse->nMem;
   sqlite3VdbeAddOp2(v, OP_Integer, 1, pMWin->regFirst);
 
@@ -2109,10 +2111,12 @@
 **       }
 **       while( !eof csrCurrent ){
 **         RETURN_ROW
 **       }
 **
+** The text above leaves out many details. Refer to the code and comments
+** below for a more complete picture.
 */
 void sqlite3WindowCodeStep(
   Parse *pParse,                  /* Parse context */
   Select *p,                      /* Rewritten SELECT statement */
   WhereInfo *pWInfo,              /* Context returned by sqlite3WhereBegin() */
@@ -2300,12 +2304,10 @@
   }
 
   sqlite3VdbeAddOp2(v, OP_Integer, 0, pMWin->regFirst);
   sqlite3VdbeAddOp2(v, OP_Goto, 0, lblWhereEnd);
 
-  /* Begin generating SECOND_ROW_CODE */
-  VdbeModuleComment((pParse->pVdbe, "Begin WindowCodeStep.SECOND_ROW"));
   sqlite3VdbeJumpHere(v, addrIfNot);
   if( regPeer ){
     windowIfNewPeer(pParse, pOrderBy, regNewPeer, regPeer, lblWhereEnd);
   }
   if( pMWin->eStart==TK_FOLLOWING ){
@@ -2352,11 +2354,10 @@
         windowCodeOp(&s, WINDOW_AGGINVERSE, regStart, 0);
         if( regEnd ) sqlite3VdbeJumpHere(v, addr);
       }
     }
   }
-  VdbeModuleComment((pParse->pVdbe, "End WindowCodeStep.SECOND_ROW"));
 
   /* End of the main input loop */
   sqlite3VdbeResolveLabel(v, lblWhereEnd);
   sqlite3WhereEnd(pWInfo);
 
@@ -2364,11 +2365,10 @@
   if( pMWin->pPartition ){
     addrInteger = sqlite3VdbeAddOp2(v, OP_Integer, 0, regFlushPart);
     sqlite3VdbeJumpHere(v, addrGosubFlush);
   }
 
-  VdbeModuleComment((pParse->pVdbe, "Begin WindowCodeStep.FLUSH"));
   addrEmpty = sqlite3VdbeAddOp1(v, OP_Rewind, csrWrite);
   if( pMWin->eEnd==TK_PRECEDING ){
     windowCodeOp(&s, WINDOW_AGGSTEP, regEnd, 0);
     windowCodeOp(&s, WINDOW_RETURN_ROW, 0, 0);
   }else if( pMWin->eStart==TK_FOLLOWING ){
@@ -2411,13 +2411,12 @@
   }
   sqlite3VdbeJumpHere(v, addrEmpty);
 
   sqlite3VdbeAddOp1(v, OP_ResetSorter, s.current.csr);
   sqlite3VdbeAddOp2(v, OP_Integer, 1, pMWin->regFirst);
-  VdbeModuleComment((pParse->pVdbe, "End WindowCodeStep.FLUSH"));
   if( pMWin->pPartition ){
     sqlite3VdbeChangeP1(v, addrInteger, sqlite3VdbeCurrentAddr(v));
     sqlite3VdbeAddOp1(v, OP_Return, regFlushPart);
   }
 }
 
 #endif /* SQLITE_OMIT_WINDOWFUNC */