SQLite

** But sqlite3SrcListShiftJoinType() later shifts the jointypes so that each
** jointype expresses the join between the table and the previous table.
**
** In the colUsed field, the high-order bit (bit 63) is set if the table
** contains more than 63 columns and the 64-th or later column is used.
*/
struct SrcList {
  u8 nSrc;        /* Number of tables or subqueries in the FROM clause */
  u8 nAlloc;      /* Number of entries allocated in a[] below */
  struct SrcList_item {
    Schema *pSchema;  /* Schema to which this item is fixed */
    char *zDatabase;  /* Name of database holding this table */
    char *zName;      /* Name of the table */
    char *zAlias;     /* The "B" part of a "A AS B" phrase.  zName is the "A" */
    Table *pTab;      /* An SQL table corresponding to zName */
    Select *pSelect;  /* A SELECT statement used in place of a table name */

** term of a join.  The WhereClause object holds a table of these
** objects using (maskSelf,prereq,) as the primary key.  Note that the
** same join term might have multiple associated WhereLoop objects.
*/
struct WhereLoop {
  Bitmask prereq;       /* Bitmask of other loops that must run first */
  Bitmask maskSelf;     /* Bitmask identifying table iTab */
  u8 iTab;              /* Position in FROM clause of table coded by this loop */
  u8 iSortIdx;          /* Sorting index number.  0==None */
  u16 nTerm;            /* Number of entries in aTerm[] */
  u32 wsFlags;          /* WHERE_* flags describing the plan */
  double rSetup;        /* One-time setup cost (ex: create transient index) */
  double rRun;          /* Cost of running each loop */
  double nOut;          /* Estimated number of output rows */
  union {
    struct {               /* Information for internal btree tables */

  struct SrcList_item *pSrc,     /* Table we are trying to access */
  Bitmask notReady               /* Tables in outer loops of the join */
){
  char aff;
  if( pTerm->leftCursor!=pSrc->iCursor ) return 0;
  if( (pTerm->eOperator & WO_EQ)==0 ) return 0;
  if( (pTerm->prereqRight & notReady)!=0 ) return 0;
  if( pTerm->u.leftColumn<0 ) return 0;
  aff = pSrc->pTab->aCol[pTerm->u.leftColumn].affinity;
  if( !sqlite3IndexAffinityOk(pTerm->pExpr, aff) ) return 0;
  return 1;
}
#endif

#ifndef SQLITE_OMIT_AUTOMATIC_INDEX

** Insert or replace a WhereLoop entry using the template supplied.
**
** An existing WhereLoop entry might be overwritten if the new template
** is better and has fewer dependencies.  Or the template will be ignored
** and no insert will occur if an existing WhereLoop is faster and has
** fewer dependencies than the template.  Otherwise a new WhereLoop is
** added based no the template.
**
** If pBuilder->pBest is not NULL then we only care about the very
** best template and that template should be stored in pBuilder->pBest.
** If pBuilder->pBest is NULL then a list of the best templates are stored
** in pBuilder->pWInfo->pLoops.
**
** When accumulating multiple loops (when pBuilder->pBest is NULL) we
** still might overwrite similar loops with the new template if the
** template is better.  Loops may be overwritten if the following 
** conditions are met:
**
**    (1)  They have the same iTab.
**    (2)  They have the same iSortIdx.
**    (3)  The template has same or fewer dependencies than the current loop
**    (4)  The template has the same or lower cost than the current loop
*/
static int whereLoopInsert(WhereLoopBuilder *pBuilder, WhereLoop *pTemplate){
  WhereLoop **ppPrev, *p, *pNext = 0, *pToFree = 0;
  WhereTerm **paTerm = 0;
  sqlite3 *db = pBuilder->db;
  WhereInfo *pWInfo = pBuilder->pWInfo;

  /* If pBuilder->pBest is defined, then only keep track of the single
  ** best WhereLoop.  pBuilder->pBest->maskSelf==0 indicates that no
  ** prior WhereLoops have been evaluated and that the current pTemplate
  ** is therefore the first and hence the best and should be retained.
  */
  if( (p = pBuilder->pBest)!=0 ){
    if( p->maskSelf!=0 ){
      if( p->rRun+p->rSetup < pTemplate->rRun+pTemplate->rSetup ){
        return SQLITE_OK;
      }
      if( p->rRun+p->rSetup == pTemplate->rRun+pTemplate->rSetup
       && p->prereq <= pTemplate->prereq ){
        return SQLITE_OK;
      }
    }
    *p = *pTemplate;
    p->aTerm = 0;
    p->u.vtab.needFree = 0;
    return SQLITE_OK;
  }

  /* Search for an existing WhereLoop to overwrite, or which takes
  ** priority over pTemplate.
  */
  for(ppPrev=&pWInfo->pLoops, p=*ppPrev; p; ppPrev=&p->pNextLoop, p=*ppPrev){
    if( p->iTab!=pTemplate->iTab || p->iSortIdx!=pTemplate->iSortIdx ) continue;
    if( (p->prereq & pTemplate->prereq)==p->prereq
     && p->rSetup<=pTemplate->rSetup
     && p->rRun<=pTemplate->rRun
    ){
      /* Already holding an equal or better WhereLoop.
      ** Return without changing or adding anything */
      return SQLITE_OK;

  p->pNextLoop = pNext;
  *ppPrev = p;
  p->aTerm = paTerm;
  if( p->nTerm ){
    memcpy(p->aTerm, pTemplate->aTerm, p->nTerm*sizeof(p->aTerm[0]));
  }
  if( (p->wsFlags & WHERE_VIRTUALTABLE)==0 ){
    if( p->u.btree.pIndex && p->u.btree.pIndex && p->u.btree.pIndex->tnum==0 ){
      p->u.btree.pIndex = 0;
    }
  }else{
    pTemplate->u.vtab.needFree = 0;
  }
  return SQLITE_OK;
}

    ){
      whereLoopAddBtreeIndex(pBuilder, pSrc, pProbe, nInMul*nIn);
    }
  }
  *pNew = savedLoop;
  return rc;
}

/*
** Return True if it is possible that pIndex might be useful in
** implementing the ORDER BY clause in pBuilder.
**
** Return False if pBuilder does not contain an ORDER BY clause or
** if there is no way for pIndex to be useful in implementing that
** ORDER BY clause.
*/
static int indexMightHelpWithOrderBy(
  WhereLoopBuilder *pBuilder,
  Index *pIndex,
  int iCursor
){
  ExprList *pOB;
  int iCol;
  int ii;

  if( (pOB = pBuilder->pOrderBy)==0 ) return 0;
  iCol = pIndex->aiColumn[0];
  for(ii=0; ii<pOB->nExpr; ii++){
    Expr *pExpr = pOB->a[ii].pExpr;
    if( pExpr->op!=TK_COLUMN ) return 0;
    if( pExpr->iTable==iCursor ){
      if( pExpr->iColumn==iCol ) return 1;
      return 0;
    }
  }
  return 0;
}

/*
** Add all WhereLoop objects a single table of the join were the table
** is idenfied by pBuilder->pNew->iTab.  That table is guaranteed to be
** a b-tree table, not a virtual table.
*/
static int whereLoopAddBtree(
  WhereLoopBuilder *pBuilder, /* WHERE clause information */
  Bitmask mExtra              /* Extra prerequesites for using this table */
){
  Index *pProbe;              /* An index we are evaluating */
  Index sPk;                  /* A fake index object for the primary key */
  tRowcnt aiRowEstPk[2];      /* The aiRowEst[] value for the sPk index */
  int aiColumnPk = -1;        /* The aColumn[] value for the sPk index */
  struct SrcList_item *pSrc;  /* The FROM clause btree term to add */
  WhereLoop *pNew;            /* Template WhereLoop object */
  int rc = SQLITE_OK;         /* Return code */
  int iSortIdx = 0;           /* Index number */
  int b;                      /* A boolean value */
  double rSize;               /* number of rows in the table */
  double rLogSize;            /* Logarithm of the number of rows in the table */
  
  pNew = pBuilder->pNew;
  pSrc = pBuilder->pTabList->a + pNew->iTab;
  assert( !IsVirtual(pSrc->pTab) );

  if( pSrc->pIndex ){
    /* An INDEXED BY clause specifies a particular index to use */
    pProbe = pSrc->pIndex;

        pNew->wsFlags = WHERE_TEMP_INDEX;
        pNew->prereq = mExtra | pTerm->prereqRight;
        rc = whereLoopInsert(pBuilder, pNew);
      }
    }
  }

  /* Loop over all indices
  */
  for(; rc==SQLITE_OK && pProbe; pProbe=pProbe->pNext, iSortIdx++){
    pNew->u.btree.nEq = 0;
    pNew->nTerm = 0;
    pNew->iSortIdx = 0;
    pNew->rSetup = (double)0;
    pNew->prereq = mExtra;
    pNew->u.btree.pIndex = pProbe;





    b = indexMightHelpWithOrderBy(pBuilder, pProbe, pSrc->iCursor);





    if( pProbe->tnum<=0 ){
      /* Integer primary key index */
      pNew->wsFlags = WHERE_IPK;

      /* Full table scan */
      pNew->nOut = rSize;
      pNew->rRun = (rSize + rLogSize)*(3+b); /* 4x penalty for a full-scan */
      rc = whereLoopInsert(pBuilder, pNew);
      if( rc ) break;
    }else{
      Bitmask m = pSrc->colUsed;
      int j;
      for(j=pProbe->nColumn-1; j>=0; j--){
        int x = pProbe->aiColumn[j];
        if( x<BMS-1 ){
          m &= ~(((Bitmask)1)<<x);
        }
      }
      pNew->wsFlags = (m==0) ? WHERE_IDX_ONLY : 0;

      /* Full scan via index */
      if( m==0 || b ){
        pNew->iSortIdx = b ? iSortIdx : 0;
        pNew->nOut = rSize;
        pNew->rRun = (m==0) ? (rSize + rLogSize)*(1+b) : (rSize*rLogSize);
        rc = whereLoopInsert(pBuilder, pNew);
        if( rc ) break;
      }
    }
    rc = whereLoopAddBtreeIndex(pBuilder, pSrc, pProbe, 1);

    /* If there was an INDEXED BY clause, then only that one index is
    ** considered. */
    if( pSrc->pIndex ) break;
  }
  return rc;

      pNew->nTerm = 1;
      pNew->aTerm[0] = pTerm;
      pNew->wsFlags = WHERE_MULTI_OR;
      pNew->rSetup = (double)0;
      pNew->rRun = rTotal;
      pNew->nOut = nRow;
      pNew->prereq = prereq;
      memset(&pNew->u, 0, sizeof(pNew->u));
      rc = whereLoopInsert(pBuilder, pNew);
    }
  }
  return rc;
}

/*