SQLite

  /* Extract the content for the p2+1-th column.  Control can only
  ** reach this point if aOffset[p2], aOffset[p2+1], and aType[p2] are
  ** all valid.
  */
  assert( p2<pC->nHdrParsed );
  assert( rc==SQLITE_OK );
  assert( sqlite3VdbeCheckMemInvariants(pDest) );
  VdbeMemReleaseExtern(pDest);
  if( pC->szRow>=aOffset[p2+1] ){
    /* This is the common case where the desired content fits on the original
    ** page - where the content is not on an overflow page */

    sqlite3VdbeSerialGet(pC->aRow+aOffset[p2], aType[p2], pDest);
  }else{
    /* This branch happens only when content is on overflow pages */
    t = aType[p2];
    if( ((pOp->p5 & (OPFLAG_LENGTHARG|OPFLAG_TYPEOFARG))!=0
          && ((t>=12 && (t&1)==0) || (pOp->p5 & OPFLAG_TYPEOFARG)!=0))
     || (len = sqlite3VdbeSerialTypeLen(t))==0

    const void *v1, *v2;
    int n1, n2;
    Mem c1;
    Mem c2;
    c1.db = c2.db = pMem1->db;
    c1.flags = c2.flags = 0;
    c1.zMalloc = c2.zMalloc = 0;
    c1.xDel = c2.xDel = 0;
    sqlite3VdbeMemShallowCopy(&c1, pMem1, MEM_Ephem);
    sqlite3VdbeMemShallowCopy(&c2, pMem2, MEM_Ephem);
    v1 = sqlite3ValueText((sqlite3_value*)&c1, pColl->enc);
    n1 = v1==0 ? 0 : c1.n;
    v2 = sqlite3ValueText((sqlite3_value*)&c2, pColl->enc);
    n2 = v2==0 ? 0 : c2.n;
    rc = pColl->xCmp(pColl->pUser, n1, v1, n2, v2);

    memcpy(pMem, &t, sizeof(t));
    rc = ctx.isError;
  }
  return rc;
}

/*
** If the memory cell contains a value that must be freed by
** invoking an external callback, then free it now. 
**
** This routine does NOT do any of the following:
**    (1)  Set the Mem.flags field to a rational value.
**    (2)  Free memory held by Mem.zMalloc
** The caller is expected to take care of setting Mem.flags appropriately.
**
** The VdbeMemReleaseExtern() macro invokes this routine if only if there
** is work for this routine to do.
*/
void sqlite3VdbeMemReleaseExternal(Mem *p){
  assert( p->db==0 || sqlite3_mutex_held(p->db->mutex) );
  if( p->flags&MEM_Agg ){

}

/*
** Release memory held by the Mem p, both external memory cleared
** by p->xDel and memory in p->zMalloc.
**
** This is a helper routine invoked by sqlite3VdbeMemRelease() in
** the uncommon case when there really is memory in p that needs
** to be freeing.
*/
static SQLITE_NOINLINE void vdbeMemRelease(Mem *p){
  if( VdbeMemDynamic(p) ){
    sqlite3VdbeMemReleaseExternal(p);
  }
  if( p->zMalloc ){
    sqlite3DbFree(p->db, p->zMalloc);
    p->zMalloc = 0;
  }
  p->z = 0;
}

/*
** Release any memory held by the Mem. This may leave the Mem.flags in an
** inconsistent state, for example with (Mem.z==0) and (Mem.flags==MEM_Str).
**
** This routine releases both the Mem.xDel space and the Mem.zMalloc space.
** Use sqlite3VdbeMemReleaseExternal() to release just the Mem.xDel space.
*/
void sqlite3VdbeMemRelease(Mem *p){
  assert( sqlite3VdbeCheckMemInvariants(p) );
  if( VdbeMemDynamic(p) || p->zMalloc ){
    vdbeMemRelease(p);
  }else{
    p->z = 0;

** and flags gets srcType (either MEM_Ephem or MEM_Static).
*/
void sqlite3VdbeMemShallowCopy(Mem *pTo, const Mem *pFrom, int srcType){
  assert( (pFrom->flags & MEM_RowSet)==0 );
  assert( pTo->db==pFrom->db );
  VdbeMemReleaseExtern(pTo);
  memcpy(pTo, pFrom, MEMCELLSIZE);
  assert( pTo->xDel==0 );
  if( (pFrom->flags&MEM_Static)==0 ){
    pTo->flags &= ~(MEM_Dyn|MEM_Static|MEM_Ephem);
    assert( srcType==MEM_Ephem || srcType==MEM_Static );
    pTo->flags |= srcType;
  }
}

/*
** Make a full copy of pFrom into pTo.  Prior contents of pTo are
** freed before the copy is made.
*/
int sqlite3VdbeMemCopy(Mem *pTo, const Mem *pFrom){
  int rc = SQLITE_OK;

  assert( pTo->db==pFrom->db );
  assert( (pFrom->flags & MEM_RowSet)==0 );
  VdbeMemReleaseExtern(pTo);
  memcpy(pTo, pFrom, MEMCELLSIZE);
  pTo->flags &= ~MEM_Dyn;
  assert( pTo->xDel==0 );

  if( pTo->flags&(MEM_Str|MEM_Blob) ){
    if( 0==(pFrom->flags&MEM_Static) ){
      pTo->flags |= MEM_Ephem;
      rc = sqlite3VdbeMemMakeWriteable(pTo);
    }
  }

  Mem *pMem         /* OUT: Return data in this Mem structure. */
){
  char *zData;        /* Data from the btree layer */
  u32 available = 0;  /* Number of bytes available on the local btree page */
  int rc = SQLITE_OK; /* Return code */

  assert( sqlite3BtreeCursorIsValid(pCur) );
  assert( pMem->xDel==0 );

  /* Note: the calls to BtreeKeyFetch() and DataFetch() below assert() 
  ** that both the BtShared and database handle mutexes are held. */
  assert( (pMem->flags & MEM_RowSet)==0 );
  if( key ){
    zData = (char *)sqlite3BtreeKeyFetch(pCur, &available);
  }else{
    zData = (char *)sqlite3BtreeDataFetch(pCur, &available);
  }
  assert( zData!=0 );

  if( offset+amt<=available ){
    sqlite3VdbeMemRelease(pMem);
    pMem->z = &zData[offset];
    pMem->flags = MEM_Blob|MEM_Ephem;
    pMem->n = (int)amt;
  }else{
    pMem->flags = MEM_Null;
    if( SQLITE_OK==(rc = sqlite3VdbeMemGrow(pMem, amt+2, 0)) ){
      if( key ){
        rc = sqlite3BtreeKey(pCur, offset, amt, pMem->z);
      }else{
        rc = sqlite3BtreeData(pCur, offset, amt, pMem->z);
      }
      if( rc==SQLITE_OK ){
        pMem->z[amt] = 0;
        pMem->z[amt+1] = 0;
        pMem->flags = MEM_Blob|MEM_Term;
        pMem->n = (int)amt;
      }else{
        sqlite3VdbeMemRelease(pMem);
      }
    }
  }

  return rc;
}

/*