SQLite

static void stat4Push(
  sqlite3_context *context,
  int argc,
  sqlite3_value **argv
){
  Stat4Accum *p = (Stat4Accum*)sqlite3_value_blob(argv[0]);
  i64 rowid = sqlite3_value_int64(argv[1]);

  struct Stat4Sample *pSample;
  u32 h;
  int iMin = p->iMin;
  int i;
  u8 isPSample = 0;
  u8 doInsert = 0;

    nSample = sqlite3_column_int(pStmt, 1);
    pIdx = sqlite3FindIndex(db, zIndex, zDb);
    if( pIdx==0 ) continue;
    assert( pIdx->nSample==0 );
    pIdx->nSample = nSample;
    nByte = sizeof(IndexSample) * nSample;
    nByte += sizeof(tRowcnt) * pIdx->nColumn * 3 * nSample;
    nByte += pIdx->nColumn * sizeof(tRowcnt);    /* Space for Index.aAvgEq[] */

    pIdx->aSample = sqlite3DbMallocZero(db, nByte);

    if( pIdx->aSample==0 ){
      sqlite3_finalize(pStmt);
      return SQLITE_NOMEM;
    }
    pSpace = (tRowcnt*)&pIdx->aSample[nSample];
    pIdx->aAvgEq = pSpace; pSpace += pIdx->nColumn;
    for(i=0; i<pIdx->nSample; i++){
      pIdx->aSample[i].anEq = pSpace; pSpace += pIdx->nColumn;
      pIdx->aSample[i].anLt = pSpace; pSpace += pIdx->nColumn;
      pIdx->aSample[i].anDLt = pSpace; pSpace += pIdx->nColumn;
    }
    assert( ((u8*)pSpace)-nByte==(u8*)(pIdx->aSample) );
  }

    nCol = pIdx->nColumn;
    decodeIntArray((char*)sqlite3_column_text(pStmt,1), nCol, pSample->anEq, 0);
    decodeIntArray((char*)sqlite3_column_text(pStmt,2), nCol, pSample->anLt, 0);
    decodeIntArray((char*)sqlite3_column_text(pStmt,3), nCol, pSample->anDLt,0);

    if( idx==pIdx->nSample-1 ){
      int iCol;
      for(iCol=0; iCol<pIdx->nColumn; iCol++){
        tRowcnt avgEq = 0;
        tRowcnt nDLt = pSample->anDLt[iCol];
        if( nDLt>idx ){
          for(i=0, sumEq=0; i<idx; i++) sumEq += pIdx->aSample[i].anEq[iCol];
          avgEq = (pSample->anLt[iCol] - sumEq)/(nDLt - idx);
        }
        if( avgEq==0 ) avgEq = 1;
        pIdx->aAvgEq[iCol] = avgEq;
      }
    }

    pSample->n = sqlite3_column_bytes(pStmt, 4);
    pSample->p = sqlite3DbMallocZero(db, pSample->n);
    if( pSample->p==0 ){
      sqlite3_finalize(pStmt);
      return SQLITE_NOMEM;

  u16 nColumn;             /* Number of columns in table used by this index */
  u8 onError;              /* OE_Abort, OE_Ignore, OE_Replace, or OE_None */
  unsigned autoIndex:2;    /* 1==UNIQUE, 2==PRIMARY KEY, 0==CREATE INDEX */
  unsigned bUnordered:1;   /* Use this index for == or IN queries only */
  unsigned uniqNotNull:1;  /* True if UNIQUE and NOT NULL for all columns */
#ifdef SQLITE_ENABLE_STAT4
  int nSample;             /* Number of elements in aSample[] */
  tRowcnt *aAvgEq;         /* Average nEq values for keys not in aSample */
  IndexSample *aSample;    /* Samples of the left-most key */
#endif
};

/*
** Each sample stored in the sqlite_stat3 table is represented in memory 
** using a structure of this type.  See documentation at the top of the

  UnpackedRecord *pRec,       /* Vector of values to consider */
  int roundUp,                /* Round up if true.  Round down if false */
  tRowcnt *aStat              /* OUT: stats written here */
){
  IndexSample *aSample = pIdx->aSample;
  int i;
  int isEq = 0;
  int iCol = pRec->nField-1;

  assert( pRec->nField>0 && iCol<pIdx->nColumn );
  for(i=0; i<pIdx->nSample; i++){
    int res = sqlite3VdbeRecordCompare(aSample[i].n, aSample[i].p, pRec);
    if( res>=0 ){
      isEq = (res==0);
      break;
    }
  }

  /* At this point, aSample[i] is the first sample that is greater than
  ** or equal to pVal.  Or if i==pIdx->nSample, then all samples are less
  ** than pVal.  If aSample[i]==pVal, then isEq==1.
  */
  if( isEq ){
    assert( i<pIdx->nSample );
    aStat[0] = aSample[i].anLt[iCol];
    aStat[1] = aSample[i].anEq[iCol];
  }else{
    tRowcnt iLower, iUpper, iGap;
    if( i==0 ){
      iLower = 0;
      iUpper = aSample[0].anLt[iCol];
    }else{
      iUpper = i>=pIdx->nSample ? pIdx->aiRowEst[0] : aSample[i].anLt[iCol];
      iLower = aSample[i-1].anEq[iCol] + aSample[i-1].anLt[iCol];
    }
    aStat[1] = pIdx->aAvgEq[iCol];
    if( iLower>=iUpper ){
      iGap = 0;
    }else{
      iGap = iUpper - iLower;
    }
    if( roundUp ){
      iGap = (iGap*2)/3;

  }

  if( nEq>p->nColumn ){
    *pnRow = 1;
    return SQLITE_OK;
  }

  aff = p->pTable->aCol[p->aiColumn[nEq-1]].affinity;
  rc = sqlite3Stat4ProbeSetValue(pParse, p, &pRec, pExpr, aff, nEq-1, &bOk);
  pBuilder->pRec = pRec;
  if( rc!=SQLITE_OK ) return rc;
  if( bOk==0 ) return SQLITE_NOTFOUND;
  pBuilder->nRecValid = nEq;

  rc = whereKeyStats(pParse, p, pRec, 0, a);
  if( rc==SQLITE_OK ){
    WHERETRACE(0x100,("equality scan regions: %d\n", (int)a[1]));
    *pnRow = a[1];
    if( pBuilder->nMaxRowcnt && *pnRow>pBuilder->nMaxRowcnt ){
      *pnRow = pBuilder->nMaxRowcnt;
    }
  }
  
  return rc;
}
#endif /* defined(SQLITE_ENABLE_STAT4) */

#ifdef SQLITE_ENABLE_STAT4
/*
** Estimate the number of rows that will be returned based on