SQLite

#define WHERE_OR_SUBCLAUSE     0x0020 /* Processing a sub-WHERE as part of
                                      ** the OR optimization  */
#define WHERE_GROUPBY          0x0040 /* pOrderBy is really a GROUP BY */
#define WHERE_DISTINCTBY       0x0080 /* pOrderby is really a DISTINCT clause */
#define WHERE_WANT_DISTINCT    0x0100 /* All output needs to be distinct */
#define WHERE_SORTBYGROUP      0x0200 /* Support sqlite3WhereIsSorted() */
#define WHERE_SEEK_TABLE       0x0400 /* Do not defer seeks on main table */
#define WHERE_ORDERBY_LIMIT    0x0800 /* ORDERBY+LIMIT on the inner loop */
                        /*     0x1000    not currently used */
                        /*     0x2000    not currently used */
#define WHERE_USE_LIMIT        0x4000 /* Use the LIMIT in cost estimates */
                        /*     0x8000    not currently used */

/* Allowed return values from sqlite3WhereIsDistinct()
*/

** the pOrderBy terms can be matched in any order.  With ORDER BY, the 
** pOrderBy terms must be matched in strict left-to-right order.
*/
static i8 wherePathSatisfiesOrderBy(
  WhereInfo *pWInfo,    /* The WHERE clause */
  ExprList *pOrderBy,   /* ORDER BY or GROUP BY or DISTINCT clause to check */
  WherePath *pPath,     /* The WherePath to check */
  u16 wctrlFlags,       /* WHERE_GROUPBY or _DISTINCTBY or _ORDERBY_LIMIT */
  u16 nLoop,            /* Number of entries in pPath->aLoop[] */
  WhereLoop *pLast,     /* Add this WhereLoop to the end of pPath->aLoop[] */
  Bitmask *pRevMask     /* OUT: Mask of WhereLoops to run in reverse order */
){
  u8 revSet;            /* True if rev is known */
  u8 rev;               /* Composite sort order */
  u8 revIdx;            /* Index sort order */
  u8 isOrderDistinct;   /* All prior WhereLoops are order-distinct */
  u8 distinctColumns;   /* True if the loop has UNIQUE NOT NULL columns */
  u8 isMatch;           /* iColumn matches a term of the ORDER BY clause */
  u16 eqOpMask;         /* Allowed equality operators */
  u16 nKeyCol;          /* Number of key columns in pIndex */
  u16 nColumn;          /* Total number of ordered columns in the index */
  u16 nOrderBy;         /* Number terms in the ORDER BY clause */
  int iLoop;            /* Index of WhereLoop in pPath being processed */
  int i, j;             /* Loop counters */
  int iCur;             /* Cursor number for current WhereLoop */
  int iColumn;          /* A column number within table iCur */

  nOrderBy = pOrderBy->nExpr;
  testcase( nOrderBy==BMS-1 );
  if( nOrderBy>BMS-1 ) return 0;  /* Cannot optimize overly large ORDER BYs */
  isOrderDistinct = 1;
  obDone = MASKBIT(nOrderBy)-1;
  orderDistinctMask = 0;
  ready = 0;
  eqOpMask = WO_EQ | WO_IS | WO_ISNULL;
  if( wctrlFlags & WHERE_ORDERBY_LIMIT ) eqOpMask |= WO_IN;
  for(iLoop=0; isOrderDistinct && obSat<obDone && iLoop<=nLoop; iLoop++){
    if( iLoop>0 ) ready |= pLoop->maskSelf;
    if( iLoop<nLoop ){
      pLoop = pPath->aLoop[iLoop];
      if( wctrlFlags & WHERE_ORDERBY_LIMIT ) continue;
    }else{
      pLoop = pLast;
    }
    if( pLoop->wsFlags & WHERE_VIRTUALTABLE ){
      if( pLoop->u.vtab.isOrdered ) obSat = obDone;
      break;
    }
    iCur = pWInfo->pTabList->a[pLoop->iTab].iCursor;

    /* Mark off any ORDER BY term X that is a column in the table of
    ** the current loop for which there is term in the WHERE
    ** clause of the form X IS NULL or X=? that reference only outer
    ** loops.
    */
    for(i=0; i<nOrderBy; i++){
      if( MASKBIT(i) & obSat ) continue;
      pOBExpr = sqlite3ExprSkipCollate(pOrderBy->a[i].pExpr);
      if( pOBExpr->op!=TK_COLUMN ) continue;
      if( pOBExpr->iTable!=iCur ) continue;
      pTerm = sqlite3WhereFindTerm(&pWInfo->sWC, iCur, pOBExpr->iColumn,
                       ~ready, eqOpMask, 0);
      if( pTerm==0 ) continue;
      if( (pTerm->eOperator&(WO_EQ|WO_IS))!=0 && pOBExpr->iColumn>=0 ){
        const char *z1, *z2;
        pColl = sqlite3ExprCollSeq(pWInfo->pParse, pOrderBy->a[i].pExpr);
        if( !pColl ) pColl = db->pDfltColl;
        z1 = pColl->zName;
        pColl = sqlite3ExprCollSeq(pWInfo->pParse, pTerm->pExpr);

      ** that are not constrained by == or IN.
      */
      rev = revSet = 0;
      distinctColumns = 0;
      for(j=0; j<nColumn; j++){
        u8 bOnce;   /* True to run the ORDER BY search loop */

        /* Skip over == and IS and ISNULL terms.
        ** (Also skip IN terms when doing WHERE_ORDERBY_LIMIT processing)
        */
        if( j<pLoop->u.btree.nEq
         && pLoop->nSkip==0
         && ((i = pLoop->aLTerm[j]->eOperator) & eqOpMask)!=0
        ){
          if( i & WO_ISNULL ){
            testcase( isOrderDistinct );
            isOrderDistinct = 0;
          }
          continue;  
        }

        pWInfo->eDistinct = WHERE_DISTINCT_ORDERED;
      }
    }else{
      pWInfo->nOBSat = pFrom->isOrdered;
      pWInfo->revMask = pFrom->revLoop;
      if( pWInfo->nOBSat<=0 ){
        pWInfo->nOBSat = 0;
        if( nLoop>0 ){
          Bitmask m;
          int rc = wherePathSatisfiesOrderBy(pWInfo, pWInfo->pOrderBy, pFrom,
                      WHERE_ORDERBY_LIMIT, nLoop-1, pFrom->aLoop[nLoop-1], &m);
          if( rc==pWInfo->pOrderBy->nExpr ){
            pWInfo->bOrderedInnerLoop = 1;
            pWInfo->revMask = m;
          }
        }
      }
    }
    if( (pWInfo->wctrlFlags & WHERE_SORTBYGROUP)
        && pWInfo->nOBSat==pWInfo->pOrderBy->nExpr && nLoop>0
    ){
      Bitmask revMask = 0;
      int nOrder = wherePathSatisfiesOrderBy(pWInfo, pWInfo->pOrderBy,