SQLite

#define BMS  ((int)(sizeof(Bitmask)*8))

/*
** A bit in a Bitmask
*/
#define MASKBIT(n)   (((Bitmask)1)<<(n))
#define MASKBIT32(n) (((unsigned int)1)<<(n))
#define ALLBITS      ((Bitmask)-1)

/*
** The following structure describes the FROM clause of a SELECT statement.
** Each table or subquery in the FROM clause is a separate element of
** the SrcList.a[] array.
**
** With the addition of multiple database support, the following structure

  chngKey = chngRowid + chngPk;

  /* The SET expressions are not actually used inside the WHERE loop.  
  ** So reset the colUsed mask. Unless this is a virtual table. In that
  ** case, set all bits of the colUsed mask (to ensure that the virtual
  ** table implementation makes all columns available).
  */
  pTabList->a[0].colUsed = IsVirtual(pTab) ? ALLBITS : 0;

  hasFK = sqlite3FkRequired(pParse, pTab, aXRef, chngKey);

  /* There is one entry in the aRegIdx[] array for each index on the table
  ** being updated.  Fill in aRegIdx[] with a register number that will hold
  ** the key for accessing each index.
  **

** before it is added to pBuilder.
**
** Output parameter *pbIn is set to true if the plan added to pBuilder
** uses one or more WO_IN terms, or false otherwise.
*/
static int whereLoopAddVirtualOne(
  WhereLoopBuilder *pBuilder,
  Bitmask mPrereq,                /* Mask of tables that must be used. */
  Bitmask mUsable,                /* Mask of usable tables */
  u16 mExclude,                   /* Exclude terms using these operators */
  sqlite3_index_info *pIdxInfo,   /* Populated object for xBestIndex */
  int *pbIn                       /* OUT: True if plan uses an IN(...) op */
){
  WhereClause *pWC = pBuilder->pWC;
  struct sqlite3_index_constraint *pIdxCons;
  struct sqlite3_index_constraint_usage *pUsage = pIdxInfo->aConstraintUsage;
  int i;

  assert( (mPrereq & mUnusable)==0 );
  pWInfo = pBuilder->pWInfo;
  pParse = pWInfo->pParse;
  pWC = pBuilder->pWC;
  pNew = pBuilder->pNew;
  pSrc = &pWInfo->pTabList->a[pNew->iTab];
  assert( IsVirtual(pSrc->pTab) );
  p = allocateIndexInfo(pParse, pWC, mUnusable, pSrc, pBuilder->pOrderBy);
  if( p==0 ) return SQLITE_NOMEM_BKPT;
  pNew->rSetup = 0;
  pNew->wsFlags = WHERE_VIRTUALTABLE;
  pNew->nLTerm = 0;
  pNew->u.vtab.needFree = 0;
  nConstraint = p->nConstraint;
  if( whereLoopResize(pParse->db, pNew, nConstraint) ){
    sqlite3DbFree(pParse->db, p);
    return SQLITE_NOMEM_BKPT;
  }

  /* First call xBestIndex() with all constraints usable. */
  rc = whereLoopAddVirtualOne(pBuilder, mPrereq, ALLBITS, 0, p, &bIn);
  mBest = pNew->prereq & ~mPrereq;

  /* If the call to xBestIndex() with all terms enabled produced a plan
  ** that does not require any source tables, there is no point in making
  ** any further calls - if the xBestIndex() method is sane they will all
  ** return the same plan anyway.
  */
  if( mBest ){
    int seenZero = 0;             /* True if a plan with no prereqs seen */
    int seenZeroNoIN = 0;         /* Plan with no prereqs and no IN(...) seen */
    Bitmask mPrev = 0;
    Bitmask mBestNoIn = 0;

    /* If the plan produced by the earlier call uses an IN(...) term, call
    ** xBestIndex again, this time with IN(...) terms disabled. */
    if( rc==SQLITE_OK && bIn ){
      rc = whereLoopAddVirtualOne(pBuilder, mPrereq, ALLBITS, WO_IN, p, &bIn);
      mBestNoIn = pNew->prereq & ~mPrereq;
      if( mBestNoIn==0 ){
        seenZero = 1;
        if( bIn==0 ) seenZeroNoIN = 1;
      }
    }

    /* Call xBestIndex once for each distinct value of (prereqRight & ~mPrereq) 
    ** in the set of terms that apply to the current virtual table.  */
    while( rc==SQLITE_OK ){
      int i;
      Bitmask mNext = ALLBITS;
      assert( mNext>0 );
      for(i=0; i<nConstraint; i++){
        Bitmask mThis = (
            pWC->a[p->aConstraint[i].iTermOffset].prereqRight & ~mPrereq
        );
        if( mThis>mPrev && mThis<mNext ) mNext = mThis;
      }
      mPrev = mNext;
      if( mNext==ALLBITS ) break;
      if( mNext==mBest || mNext==mBestNoIn ) continue;
      rc = whereLoopAddVirtualOne(pBuilder, mPrereq, mNext|mPrereq, 0, p, &bIn);
      if( pNew->prereq==mPrereq ){
        seenZero = 1;
        if( bIn==0 ) seenZeroNoIN = 1;
      }
    }

    if( db->mallocFailed ) goto whereBeginError;
    if( pWInfo->pOrderBy ){
       wherePathSolver(pWInfo, pWInfo->nRowOut+1);
       if( db->mallocFailed ) goto whereBeginError;
    }
  }
  if( pWInfo->pOrderBy==0 && (db->flags & SQLITE_ReverseOrder)!=0 ){
     pWInfo->revMask = ALLBITS;
  }
  if( pParse->nErr || NEVER(db->mallocFailed) ){
    goto whereBeginError;
  }
#ifdef WHERETRACE_ENABLED
  if( sqlite3WhereTrace ){
    sqlite3DebugPrintf("---- Solution nRow=%d", pWInfo->nRowOut);