SQLite

** Three SQL functions - stat_init(), stat_push(), and stat_get() -
** share an instance of the following structure to hold their state
** information.
*/
typedef struct StatAccum StatAccum;
typedef struct StatSample StatSample;
struct StatSample {

  tRowcnt *anDLt;                 /* sqlite_stat4.nDLt */
#ifdef SQLITE_ENABLE_STAT4
  tRowcnt *anEq;                  /* sqlite_stat4.nEq */
  tRowcnt *anLt;                  /* sqlite_stat4.nLt */
  union {
    i64 iRowid;                     /* Rowid in main table of the key */
    u8 *aRowid;                     /* Key for WITHOUT ROWID tables */
  } u;
  u32 nRowid;                     /* Sizeof aRowid[] */
  u8 isPSample;                   /* True if a periodic sample */

  nColUp = sizeof(tRowcnt)<8 ? (nCol+1)&~1 : nCol;
  nKeyCol = sqlite3_value_int(argv[1]);
  assert( nKeyCol<=nCol );
  assert( nKeyCol>0 );

  /* Allocate the space required for the StatAccum object */
  n = sizeof(*p) 

    + sizeof(tRowcnt)*nColUp;                    /* StatAccum.anDLt */
#ifdef SQLITE_ENABLE_STAT4
  n += sizeof(tRowcnt)*nColUp;                   /* StatAccum.anEq */
  if( mxSample ){
    n += sizeof(tRowcnt)*nColUp                  /* StatAccum.anLt */
      + sizeof(StatSample)*(nCol+mxSample)       /* StatAccum.aBest[], a[] */
      + sizeof(tRowcnt)*3*nColUp*(nCol+mxSample);
  }
#endif
  p = sqlite3DbMallocZero(db, n);
  if( p==0 ){
    sqlite3_result_error_nomem(context);
    return;
  }

  p->db = db;
  p->nEst = sqlite3_value_int64(argv[2]);
  p->nRow = 0;
  p->nLimit = sqlite3_value_int64(argv[3]);
  p->nCol = nCol;
  p->nKeyCol = nKeyCol;
  p->nSkipAhead = 0;
  p->current.anDLt = (tRowcnt*)&p[1];


#ifdef SQLITE_ENABLE_STAT4
  p->current.anEq = &p->current.anDLt[nColUp];
  p->mxSample = p->nLimit==0 ? mxSample : 0;
  if( mxSample ){
    u8 *pSpace;                     /* Allocated space not yet assigned */
    int i;                          /* Used to iterate through p->aSample[] */

    p->iGet = -1;
    p->nPSample = (tRowcnt)(p->nEst/(mxSample/3+1) + 1);

  UNUSED_PARAMETER( argc );
  UNUSED_PARAMETER( context );
  assert( p->nCol>0 );
  assert( iChng<p->nCol );

  if( p->nRow==0 ){
    /* This is the first call to this function. Do initialization. */
#ifdef SQLITE_ENABLE_STAT4
    for(i=0; i<p->nCol; i++) p->current.anEq[i] = 1;
#endif
  }else{
    /* Second and subsequent calls get processed here */
#ifdef SQLITE_ENABLE_STAT4
    if( p->mxSample ) samplePushPrevious(p, iChng);
#endif

    /* Update anDLt[], anLt[] and anEq[] to reflect the values that apply
    ** to the current row of the index. */
#ifdef SQLITE_ENABLE_STAT4
    for(i=0; i<iChng; i++){
      p->current.anEq[i]++;
    }
#endif
    for(i=iChng; i<p->nCol; i++){
      p->current.anDLt[i]++;
#ifdef SQLITE_ENABLE_STAT4
      if( p->mxSample ) p->current.anLt[i] += p->current.anEq[i];

      p->current.anEq[i] = 1;
#endif
    }
  }

  p->nRow++;
#ifdef SQLITE_ENABLE_STAT4
  if( p->mxSample ){
    tRowcnt nLt;

    sqlite3_str_appendf(&sStat, "%llu", 
        p->nSkipAhead ? (u64)p->nEst : (u64)p->nRow);
    for(i=0; i<p->nKeyCol; i++){
      u64 nDistinct = p->current.anDLt[i] + 1;
      u64 iVal = (p->nRow + nDistinct - 1) / nDistinct;
      if( iVal==2 && p->nRow*10 <= nDistinct*11 ) iVal = 1;
      sqlite3_str_appendf(&sStat, " %llu", iVal);
#ifdef SQLITE_ENABLE_STAT4
      assert( p->current.anEq[i] );
#endif
    }
    sqlite3ResultStrAccum(context, &sStat);
  }
#ifdef SQLITE_ENABLE_STAT4
  else if( eCall==STAT_GET_ROWID ){
    if( p->iGet<0 ){
      samplePushPrevious(p, 0);

      else if( sqlite3_strglob("costmult=[0-9]*",z)==0 ){
        pIndex->pTable->costMult = sqlite3LogEst(sqlite3Atoi(z+9));
      }
#endif
      while( z[0]!=0 && z[0]!=' ' ) z++;
      while( z[0]==' ' ) z++;
    }

    /* Set the bLowQual flag if the peak number of rows obtained
    ** from a full equality match is so large that a full table scan
    ** seems likely to be faster than using the index.
    */
    if( aLog[0] > 66              /* Index has more than 100 rows */
     && aLog[0] <= aLog[nOut-1]   /* And only a single value seen */
    ){
      pIndex->bLowQual = 1;
    }
  }
}

/*
** This callback is invoked once for each index when reading the
** sqlite_stat1 table.  
**

  unsigned idxType:2;      /* 0:Normal 1:UNIQUE, 2:PRIMARY KEY, 3:IPK */
  unsigned bUnordered:1;   /* Use this index for == or IN queries only */
  unsigned uniqNotNull:1;  /* True if UNIQUE and NOT NULL for all columns */
  unsigned isResized:1;    /* True if resizeIndexObject() has been called */
  unsigned isCovering:1;   /* True if this is a covering index */
  unsigned noSkipScan:1;   /* Do not try to use skip-scan if true */
  unsigned hasStat1:1;     /* aiRowLogEst values come from sqlite_stat1 */
  unsigned bLowQual:1;     /* sqlite_stat1 says this is a low-quality index */
  unsigned bNoQuery:1;     /* Do not use this index to optimize queries */
  unsigned bAscKeyBug:1;   /* True if the bba7b69f9849b5bf bug applies */
  unsigned bHasVCol:1;     /* Index references one or more VIRTUAL columns */
  unsigned bHasExpr:1;     /* Index contains an expression, either a literal
                           ** expression, or a reference to a VIRTUAL column */
#ifdef SQLITE_ENABLE_STAT4
  int nSample;             /* Number of elements in aSample[] */

  assert( (pNew->wsFlags & WHERE_TOP_LIMIT)==0 );
  if( pNew->wsFlags & WHERE_BTM_LIMIT ){
    opMask = WO_LT|WO_LE;
  }else{
    assert( pNew->u.btree.nBtm==0 );
    opMask = WO_EQ|WO_IN|WO_GT|WO_GE|WO_LT|WO_LE|WO_ISNULL|WO_IS;
  }
  if( pProbe->bUnordered || pProbe->bLowQual ){
    if( pProbe->bUnordered ) opMask &= ~(WO_GT|WO_GE|WO_LT|WO_LE);
    if( pProbe->bLowQual )   opMask &= ~(WO_EQ|WO_IN|WO_IS);
  }

  assert( pNew->u.btree.nEq<pProbe->nColumn );
  assert( pNew->u.btree.nEq<pProbe->nKeyCol
       || pProbe->idxType!=SQLITE_IDXTYPE_PRIMARYKEY );

  saved_nEq = pNew->u.btree.nEq;
  saved_nBtm = pNew->u.btree.nBtm;