1 files changed, 82 insertions, 0 deletions

diff --git a/src/btree.c b/src/btree.c
index 700138efc..0a8c17221 100644
--- a/src/btree.c
+++ b/src/btree.c
@@ -8152,6 +8152,83 @@ static int balance(BtCursor *pCur){
   return rc;
 }
 
+/* Overwrite content from pX into pDest.  Only do the write if the
+** content is different from what is already there.
+*/
+static int btreeOverwriteContent(
+  MemPage *pPage,           /* MemPage on which writing will occur */
+  u8 *pDest,                /* Pointer to the place to start writing */
+  const BtreePayload *pX,   /* Source of data to write */
+  int iOffset,              /* Offset of first byte to write */
+  int iAmt                  /* Number of bytes to be written */
+){
+  int nData = pX->nData - iOffset;
+  if( nData<=0 ){
+    /* Overwritting with zeros */
+    int i;
+    for(i=0; i<iAmt && pDest[i]==0; i++){}
+    if( i<iAmt ){
+      int rc = sqlite3PagerWrite(pPage->pDbPage);
+      if( rc ) return rc;
+      memset(pDest + i, 0, iAmt - i);
+    }
+  }else{
+    if( nData<iAmt ){
+      /* Mixed read data and zeros at the end.  Make a recursive call
+      ** to write the zeros then fall through to write the real data */
+      btreeOverwriteContent(pPage, pDest+nData, pX, iOffset+nData, iAmt-nData);
+      iAmt = nData;
+    }
+    if( memcmp(pDest, ((u8*)pX->pData) + iOffset, iAmt)!=0 ){
+      int rc = sqlite3PagerWrite(pPage->pDbPage);
+      if( rc ) return rc;
+      memcpy(pDest, ((u8*)pX->pData) + iOffset, iAmt);
+    }
+  }
+  return SQLITE_OK;
+}
+
+/*
+** Overwrite the cell that cursor pCur is pointing to with fresh content
+** contained in pX.
+*/
+static int btreeOverwriteCell(BtCursor *pCur, const BtreePayload *pX){
+  int iOffset;                        /* Next byte of pX->pData to write */
+  int nTotal = pX->nData + pX->nZero; /* Total bytes of to write */
+  int rc;                             /* Return code */
+  MemPage *pPage = pCur->pPage;       /* Page being written */
+  BtShared *pBt;                      /* Btree */
+  Pgno ovflPgno;                      /* Next overflow page to write */
+  u32 ovflPageSize;                   /* Size to write on overflow page */
+
+  /* Overwrite the local portion first */
+  rc = btreeOverwriteContent(pPage, pCur->info.pPayload, pX,
+                             0, pCur->info.nLocal);
+  if( rc ) return rc;
+  if( pCur->info.nLocal==nTotal ) return SQLITE_OK;
+
+  /* Now overwrite the overflow pages */
+  iOffset = pCur->info.nLocal;
+  ovflPgno = get4byte(pCur->info.pPayload + iOffset);
+  pBt = pPage->pBt;
+  ovflPageSize = pBt->usableSize - 4;
+  do{
+    rc = btreeGetPage(pBt, ovflPgno, &pPage, 0);
+    if( rc ) return rc;
+    if( iOffset+ovflPageSize>nTotal ){
+      ovflPgno = get4byte(pPage->aData + ovflPageSize);
+    }else{
+      ovflPageSize = nTotal - iOffset;
+    }
+    rc = btreeOverwriteContent(pPage, pPage->aData, pX,
+                               iOffset, ovflPageSize);
+    if( rc ) return rc;
+    iOffset += ovflPageSize;
+    sqlite3PagerUnref(pPage->pDbPage);
+  }while( iOffset<nTotal );
+  return SQLITE_OK;    
+}
+
 
 /*
 ** Insert a new record into the BTree.  The content of the new record
@@ -8250,6 +8327,11 @@ int sqlite3BtreeInsert(
     ** new row onto the end, set the "loc" to avoid an unnecessary
     ** btreeMoveto() call */
     if( (pCur->curFlags&BTCF_ValidNKey)!=0 && pX->nKey==pCur->info.nKey ){
+      /* The current is currently pointing to the entry that is to be
+      ** overwritten */
+      if( pCur->info.nPayload==pX->nData+pX->nZero ){
+        return btreeOverwriteCell(pCur, pX);
+      }
       loc = 0;
     }else if( loc==0 ){
       rc = sqlite3BtreeMovetoUnpacked(pCur, 0, pX->nKey, flags!=0, &loc);