SQLite

  int nKeyCol;              /* Number of index columns w/o the pk/rowid */
  int mxSample;             /* Maximum number of samples to accumulate */
  Stat4Sample current;      /* Current row as a Stat4Sample */
  u32 iPrn;                 /* Pseudo-random number used for sampling */
  Stat4Sample *aBest;       /* Array of nCol best samples */
  int iMin;                 /* Index in a[] of entry with minimum score */
  int nSample;              /* Current number of samples */
  int nMaxEqZero;           /* Max leading 0 in anEq[] for any a[] entry */
  int iGet;                 /* Index of current sample accessed by stat_get() */
  Stat4Sample *a;           /* Array of mxSample Stat4Sample objects */
  sqlite3 *db;              /* Database connection, for malloc() */
};

/* Reclaim memory used by a Stat4Sample
*/

** Copy the contents of sample *pNew into the p->a[] array. If necessary,
** remove the least desirable sample from p->a[] to make room.
*/
static void sampleInsert(Stat4Accum *p, Stat4Sample *pNew, int nEqZero){
  Stat4Sample *pSample = 0;
  int i;

  /* Stat4Accum.nMaxEqZero is set to the maximum number of leading 0
  ** values in the anEq[] array of any sample in Stat4Accum.a[]. In
  ** other words, if nMaxEqZero is n, then it is guaranteed that there
  ** are no samples with Stat4Sample.anEq[m]==0 for (m>=n). */
  assert( IsStat4 || nEqZero==0 );
  if( nEqZero>p->nMaxEqZero ){
    p->nMaxEqZero = nEqZero;
  }

#ifdef SQLITE_ENABLE_STAT4
  if( pNew->isPSample==0 ){
    Stat4Sample *pUpgrade = 0;
    assert( pNew->anEq[pNew->iCol]>0 );

    /* This sample is being added because the prefix that ends in column 

    Stat4Sample *pBest = &p->aBest[i];
    pBest->anEq[i] = p->current.anEq[i];
    if( p->nSample<p->mxSample || sampleIsBetter(p, pBest, &p->a[p->iMin]) ){
      sampleInsert(p, pBest, i);
    }
  }

  /* Check that no sample contains an anEq[] entry with an index of
  ** p->nMaxEqZero or greater set to zero. */
  for(i=p->nSample-1; i>=0; i--){
    int j;
    for(j=p->nMaxEqZero; j<p->nCol; j++) assert( p->a[i].anEq[j]>0 );
  }

  /* Update the anEq[] fields of any samples already collected. */
  if( iChng<p->nMaxEqZero ){
    for(i=p->nSample-1; i>=0; i--){
      int j;
      for(j=iChng; j<p->nCol; j++){
        if( p->a[i].anEq[j]==0 ) p->a[i].anEq[j] = p->current.anEq[j];
      }
    }
    p->nMaxEqZero = iChng;
  }
#endif

#if defined(SQLITE_ENABLE_STAT3) && !defined(SQLITE_ENABLE_STAT4)
  if( iChng==0 ){
    tRowcnt nLt = p->current.anLt[0];
    tRowcnt nEq = p->current.anEq[0];