SQLite

  int iCur,               /* Cursor to scan for */
  int iColumn,            /* Column to scan for */
  u32 opMask,             /* Operator(s) to scan for */
  Index *pIdx             /* Must be compatible with this index */
){
  int j;

  /* memset(pScan, 0, sizeof(*pScan)); */
  pScan->pCurrent = 0;
  pScan->pOrigWC = pWC;
  pScan->pWC = pWC;
  if( pIdx && iColumn>=0 ){
    pScan->idxaff = pIdx->pTable->aCol[iColumn].affinity;
    for(j=0; pIdx->aiColumn[j]!=iColumn; j++){
      if( NEVER(j>=pIdx->nColumn) ) return 0;
    }
    pScan->zCollName = pIdx->azColl[j];
  }else{
    pScan->idxaff = 0;
    pScan->zCollName = 0;
  }
  pScan->opMask = opMask;
  pScan->k = 0;
  pScan->aEquiv[0] = iCur;
  pScan->aEquiv[1] = iColumn;
  pScan->nEquiv = 2;
  pScan->iEquiv = 2;
  return whereScanNext(pScan);
}

** attempts to find the lowest cost path that visits each WhereLoop
** once.  This path is then loaded into the pWInfo->a[].pWLoop fields.
**
** Return SQLITE_OK on success or SQLITE_NOMEM of a memory allocation
** error occurs.
*/
static int wherePathSolver(WhereInfo *pWInfo, double nRowEst){
  int mxChoice = 10;        /* Maximum number of simultaneous paths tracked */
  int nLoop;                /* Number of terms in the join */
  sqlite3 *db;              /* The database connection */
  int iLoop;                /* Loop counter over the terms of the join */
  int ii, jj;               /* Loop counters */
  double rCost;             /* Cost of a path */
  double mxCost;            /* Maximum cost of a set of paths */
  double rSortCost;         /* Cost to do a sort */
  int nTo, nFrom;           /* Number of valid entries in aTo[] and aFrom[] */
  WherePath *aFrom;         /* All nFrom paths at the previous level */
  WherePath *aTo;           /* The nTo best paths at the current level */
  WherePath *pFrom;         /* An element of aFrom[] that we are working on */
  WherePath *pTo;           /* An element of aTo[] that we are working on */
  WhereLoop *pWLoop;        /* One of the WhereLoop objects */
  WhereLoop **pX;           /* Used to divy up the pSpace memory */
  char *pSpace;             /* Temporary memory used by this routine */

  db = pWInfo->pParse->db;
  nLoop = pWInfo->nLevel;
  mxChoice = (nLoop==1) ? 1 : (nLoop==2 ? 5 : 10);
  assert( nLoop<=pWInfo->pTabList->nSrc );
#ifdef WHERETRACE_ENABLED
  if( sqlite3WhereTrace>=2 ) sqlite3DebugPrintf("---- begin solver\n");
#endif

  /* Allocate and initialize space for aTo and aFrom */
  ii = (sizeof(WherePath)+sizeof(WhereLoop*)*nLoop)*mxChoice*2;
  pSpace = sqlite3DbMallocRaw(db, ii);
  if( pSpace==0 ) return SQLITE_NOMEM;
  aTo = (WherePath*)pSpace;
  aFrom = aTo+mxChoice;
  memset(aFrom, 0, sizeof(aFrom[0]));
  pX = (WhereLoop**)(aFrom+mxChoice);
  for(ii=mxChoice*2, pFrom=aTo; ii>0; ii--, pFrom++, pX += nLoop){
    pFrom->aLoop = pX;
  }

  /* Seed the search with a single WherePath containing zero WhereLoops */
  aFrom[0].nRow = (double)1;
  nFrom = 1;