SQLite

** Otherwise, return SQLITE_OK and set *pRes to a negative, zero or positive
** value, depending on whether key1 is smaller, equal to or larger than key2.
**
** If the bOmitRowid argument is non-zero, assume both keys end in a rowid
** field. For the purposes of the comparison, ignore it. Also, if bOmitRowid
** is true and key1 contains even a single NULL value, it is considered to
** be less than key2. Even if key2 also contains NULL values.
**
** If pKey2 is passed a NULL pointer, then it is assumed that the pCsr->aSpace
** has been allocated and contains an unpacked record that is used as key2.
*/
static int vdbeSorterCompare(
  VdbeCursor *pCsr,               /* Cursor object (for pKeyInfo) */
  int bOmitRowid,                 /* Ignore rowid field at end of keys */
  void *pKey1, int nKey1,         /* Left side of comparison */
  void *pKey2, int nKey2,         /* Right side of comparison */
  int *pRes                       /* OUT: Result of comparison */

    nSpace = ROUND8(sizeof(UnpackedRecord))+(pKeyInfo->nField+1)*sizeof(Mem);
    aSpace = (char *)sqlite3Malloc(nSpace);
    if( aSpace==0 ) return SQLITE_NOMEM;
    pSorter->aSpace = aSpace;
    pSorter->nSpace = nSpace;
  }

  if( pKey2 ){
    /* This call cannot fail. As the memory is already allocated. */
    r2 = sqlite3VdbeRecordUnpack(pKeyInfo, nKey2, pKey2, aSpace, nSpace);
    assert( r2 && (r2->flags & UNPACKED_NEED_FREE)==0 );
    assert( r2==aSpace );
  }else{
    r2 = (UnpackedRecord *)aSpace;
    assert( !bOmitRowid );
  }

  if( bOmitRowid ){
    for(i=0; i<r2->nField-1; i++){
      if( r2->aMem[i].flags & MEM_Null ){
        *pRes = -1;

        return SQLITE_OK;
      }
    }
    r2->flags |= UNPACKED_PREFIX_MATCH;
    r2->nField--;
    assert( r2->nField>0 );
  }

  *pRes = sqlite3VdbeRecordCompare(nKey1, pKey1, r2);

  return SQLITE_OK;
}

/*
** This function is called to compare two iterator keys when merging 
** multiple b-tree segments. Parameter iOut is the index of the aTree[] 
** value to recalculate.

  SorterRecord *p1,               /* First list to merge */
  SorterRecord *p2,               /* Second list to merge */
  SorterRecord **ppOut            /* OUT: Head of merged list */
){
  int rc = SQLITE_OK;
  SorterRecord *pFinal = 0;
  SorterRecord **pp = &pFinal;
  int bKey2InSpace = 0;           /* True if pCsr->aSpace contains key2 */

  while( p1 || p2 ){
    if( p1==0 ){
      *pp = p2;
      p2 = 0;
    }else if( p2==0 ){
      *pp = p1;
      p1 = 0;
    }else{
      int res;
      rc = vdbeSorterCompare(pCsr, 0, 
          p1->pVal, p1->nVal, (bKey2InSpace ? 0 : p2->pVal), p2->nVal, &res
      );
      if( rc!=SQLITE_OK ){
        vdbeSorterRecordFree(db, p1);
        vdbeSorterRecordFree(db, p2);
        vdbeSorterRecordFree(db, pFinal);
        pFinal = 0;
        break;
      }
      if( res<=0 ){
        *pp = p1;
        pp = &p1->pNext;
        p1 = p1->pNext;
        bKey2InSpace = 1;
      }else{
        *pp = p2;
        pp = &p2->pNext;
        p2 = p2->pNext;
        bKey2InSpace = 0;
      }
      *pp = 0;
    }
  }

  *ppOut = pFinal;
  return rc;