SQLite

    pColl = sqlite3ExprCollSeq(pParse, pLeft);
    if( !pColl ){
      pColl = sqlite3ExprCollSeq(pParse, pRight);
    }
  }
  return pColl;
}

/*
** Return true if CollSeq is the default built-in BINARY.
*/
int sqlite3IsBinary(const CollSeq *p){
  if( p==0 ) return 1;
  if( sqlite3_stricmp(p->zName,"BINARY")==0 ) return 1;
  return 0;
}

/*
** Generate code for a comparison operator.
*/
static int codeCompare(
  Parse *pParse,    /* The parsing (and code generating) context */
  Expr *pLeft,      /* The left operand */

    case TK_COLUMN:
    case TK_AGG_FUNCTION:
    case TK_AGG_COLUMN:
      testcase( pExpr->op==TK_ID );
      testcase( pExpr->op==TK_COLUMN );
      testcase( pExpr->op==TK_AGG_FUNCTION );
      testcase( pExpr->op==TK_AGG_COLUMN );
      if( ExprHasProperty(pExpr, EP_FixedCol) ){
        return WRC_Continue;
      }
      if( pWalker->eCode==3 && pExpr->iTable==pWalker->u.iCur ){
        return WRC_Continue;
      }
      /* Fall through */
    case TK_IF_NULL_ROW:
    case TK_REGISTER:
      testcase( pExpr->op==TK_REGISTER );

  /* Check if pExpr is identical to any GROUP BY term. If so, consider
  ** it constant.  */
  for(i=0; i<pGroupBy->nExpr; i++){
    Expr *p = pGroupBy->a[i].pExpr;
    if( sqlite3ExprCompare(0, pExpr, p, -1)<2 ){
      CollSeq *pColl = sqlite3ExprNNCollSeq(pWalker->pParse, p);
      if( sqlite3IsBinary(pColl) ){
        return WRC_Prune;
      }
    }
  }

  /* Check if pExpr is a sub-select. If so, consider it variable. */
  if( ExprHasProperty(pExpr, EP_xIsSelect) ){

                              pCol->iSorterColumn, target);
        return target;
      }
      /* Otherwise, fall thru into the TK_COLUMN case */
    }
    case TK_COLUMN: {
      int iTab = pExpr->iTable;
      if( ExprHasProperty(pExpr, EP_FixedCol) ){
        pExpr = pExpr->pLeft;
        goto expr_code_doover;
      }
      if( iTab<0 ){
        if( pParse->iSelfTab<0 ){
          /* Generating CHECK constraints or inserting into partial index */
          return pExpr->iColumn - pParse->iSelfTab;
        }else{
          /* Coding an expression that is part of an index where column names
          ** in the index refer to the table to which the index belongs */

    return 2;
  }
  if( pA->op!=TK_COLUMN && pA->op!=TK_AGG_COLUMN && pA->u.zToken ){
    if( pA->op==TK_FUNCTION ){
      if( sqlite3StrICmp(pA->u.zToken,pB->u.zToken)!=0 ) return 2;
    }else if( pA->op==TK_COLLATE ){
      if( sqlite3_stricmp(pA->u.zToken,pB->u.zToken)!=0 ) return 2;
    }else if( strcmp(pA->u.zToken,pB->u.zToken)!=0 ){
      return 2;
    }
  }
  if( (pA->flags & EP_Distinct)!=(pB->flags & EP_Distinct) ) return 2;
  if( ALWAYS((combinedFlags & EP_TokenOnly)==0) ){
    if( combinedFlags & EP_xIsSelect ) return 2;
    if( (combinedFlags & EP_FixedCol)==0
     && sqlite3ExprCompare(pParse, pA->pLeft, pB->pLeft, iTab) ) return 2;
    if( sqlite3ExprCompare(pParse, pA->pRight, pB->pRight, iTab) ) return 2;
    if( sqlite3ExprListCompare(pA->x.pList, pB->x.pList, iTab) ) return 2;
    assert( (combinedFlags & EP_Reduced)==0 );
    if( pA->op!=TK_STRING && pA->op!=TK_TRUEFALSE ){
      if( pA->iColumn!=pB->iColumn ) return 2;
      if( pA->iTable!=pB->iTable 
       && (pA->iTable!=iTab || NEVER(pB->iTable>=0)) ) return 2;

#endif

  return 1;
}
#endif /* !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW) */

/*
** A structure to keep track of all of the column values that fixed to
** a known value due to WHERE clause constraints of the form COLUMN=VALUE.
*/
typedef struct WhereConst WhereConst;
struct WhereConst {
  Parse *pParse;   /* Parsing context */
  int nConst;      /* Number for COLUMN=CONSTANT terms */
  int nChng;       /* Number of times a constant is propagated */
  Expr **apExpr;   /* [i*2] is COLUMN and [i*2+1] is VALUE */
};

/*
** Add a new entry to the pConst object
*/
static void constInsert(
  WhereConst *pConst,
  Expr *pColumn,
  Expr *pValue
){
  pConst->nConst++;
  pConst->apExpr = sqlite3DbReallocOrFree(pConst->pParse->db, pConst->apExpr,
                         pConst->nConst*2*sizeof(Expr*));
  if( pConst->apExpr==0 ){
    pConst->nConst = 0;
  }else{
    if( ExprHasProperty(pValue, EP_FixedCol) ) pValue = pValue->pLeft;
    pConst->apExpr[pConst->nConst*2-2] = pColumn;
    pConst->apExpr[pConst->nConst*2-1] = pValue;
  }
}

/*
** Find all terms of COLUMN=VALUE or VALUE=COLUMN in pExpr where VALUE
** is a constant expression and where the term must be true because it
** is part of the AND-connected terms of the expression.  For each term
** found, add it to the pConst structure.
*/
static void findConstInWhere(WhereConst *pConst, Expr *pExpr){
  Expr *pRight, *pLeft;
  CollSeq *pColl;
  if( pExpr==0 ) return;
  if( ExprHasProperty(pExpr, EP_FromJoin) ) return;
  if( pExpr->op==TK_AND ){
    findConstInWhere(pConst, pExpr->pRight);
    findConstInWhere(pConst, pExpr->pLeft);
    return;
  }
  if( pExpr->op!=TK_EQ ) return;
  pRight = pExpr->pRight;
  pLeft = pExpr->pLeft;
  assert( pRight!=0 );
  assert( pLeft!=0 );
  pColl = sqlite3BinaryCompareCollSeq(pConst->pParse, pLeft, pRight);
  if( !sqlite3IsBinary(pColl) ) return;
  if( pRight->op==TK_COLUMN
   && !ExprHasProperty(pRight, EP_FixedCol)
   && sqlite3ExprIsConstant(pLeft)
  ){
    constInsert(pConst, pRight, pLeft);
  }else
  if( pLeft->op==TK_COLUMN
   && !ExprHasProperty(pLeft, EP_FixedCol)
   && sqlite3ExprIsConstant(pRight) ){
    constInsert(pConst, pLeft, pRight);
  }
}

/*
** This is a Walker expression callback.  pExpr is a candidate expression
** to be replaced by a value.  If pExpr is equivalent to one of the
** columns named in pWalker->u.pConst, then overwrite it with its
** corresponding value.
*/
static int propagateConstantExprRewrite(Walker *pWalker, Expr *pExpr){
  int i;
  WhereConst *pConst;
  if( pExpr->op!=TK_COLUMN ) return WRC_Continue;
  if( ExprHasProperty(pExpr, EP_FixedCol) ) return WRC_Continue;
  pConst = pWalker->u.pConst;
  for(i=0; i<pConst->nConst; i++){
    Expr *pColumn = pConst->apExpr[i*2];
    if( pColumn==pExpr ) continue;
    if( pColumn->iTable!=pExpr->iTable ) continue;
    if( pColumn->iColumn!=pExpr->iColumn ) continue;
    /* A match is found.  Add the EP_FixedCol property */
    pConst->nChng++;
    ExprClearProperty(pExpr, EP_Leaf);
    ExprSetProperty(pExpr, EP_FixedCol);
    assert( pExpr->pLeft==0 );
    pExpr->pLeft = sqlite3ExprDup(pConst->pParse->db, pConst->apExpr[i*2+1], 0);
    break;
  }
  return WRC_Prune;
}

/*
** The WHERE-clause constant propagation optimization.
**
** If the WHERE clause contains terms of the form COLUMN=CONSTANT or
** CONSTANT=COLUMN that must be tree (in other words, if the terms top-level
** AND-connected terms that are not part of a ON clause from a LEFT JOIN)
** then throughout the query replace all other occurrences of COLUMN
** with CONSTANT within the WHERE clause.
**
** For example, the query:
**
**      SELECT * FROM t1, t2, t3 WHERE t1.a=39 AND t2.b=t1.a AND t3.c=t2.b
**
** Is transformed into
**
**      SELECT * FROM t1, t2, t3 WHERE t1.a=39 AND t2.b=39 AND t3.c=39
**
** Return true if any transformations where made and false if not.
**
** Implementation note:  Constant propagation is tricky due to affinity
** and collating sequence interactions.  Consider this example:
**
**    CREATE TABLE t1(a INT,b TEXT);
**    INSERT INTO t1 VALUES(123,'0123');
**    SELECT * FROM t1 WHERE a=123 AND b=a;
**    SELECT * FROM t1 WHERE a=123 AND b=123;
**
** The two SELECT statements above should return different answers.  b=a
** is alway true because the comparison uses numeric affinity, but b=123
** is false because it uses text affinity and '0123' is not the same as '123'.
** To work around this, the expression tree is not actually changed from
** "b=a" to "b=123" but rather the "a" in "b=a" is tagged with EP_FixedCol
** and the "123" value is hung off of the pLeft pointer.  Code generator
** routines know to generate the constant "123" instead of looking up the
** column value.  Also, to avoid collation problems, this optimization is
** only attempted if the "a=123" term uses the default BINARY collation.
*/
static int propagateConstants(
  Parse *pParse,   /* The parsing context */
  Select *p        /* The query in which to propagate constants */
){
  WhereConst x;
  Walker w;
  int nChng = 0;
  x.pParse = pParse;
  do{
    x.nConst = 0;
    x.nChng = 0;
    x.apExpr = 0;
    findConstInWhere(&x, p->pWhere);
    if( x.nConst ){
      memset(&w, 0, sizeof(w));
      w.pParse = pParse;
      w.xExprCallback = propagateConstantExprRewrite;
      w.xSelectCallback = sqlite3SelectWalkNoop;
      w.xSelectCallback2 = 0;
      w.walkerDepth = 0;
      w.u.pConst = &x;
      sqlite3WalkExpr(&w, p->pWhere);
      sqlite3DbFree(x.pParse->db, x.apExpr);
      nChng += x.nChng;
    }
  }while( x.nChng );  
  return nChng;
}

#if !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW)

/*
** The following are the meanings of bits in the Expr.flags field.
*/
#define EP_FromJoin  0x000001 /* Originates in ON/USING clause of outer join */
#define EP_Agg       0x000002 /* Contains one or more aggregate functions */
#define EP_HasFunc   0x000004 /* Contains one or more functions of any kind */
#define EP_FixedCol  0x000008 /* TK_Column with a known fixed value */
#define EP_Distinct  0x000010 /* Aggregate function with DISTINCT keyword */
#define EP_VarSelect 0x000020 /* pSelect is correlated, not constant */
#define EP_DblQuoted 0x000040 /* token.z was originally in "..." */
#define EP_InfixFunc 0x000080 /* True for an infix function: LIKE, GLOB, etc */
#define EP_Collate   0x000100 /* Tree contains a TK_COLLATE operator */
#define EP_Generic   0x000200 /* Ignore COLLATE or affinity on this tree */
#define EP_IntValue  0x000400 /* Integer value contained in u.iValue */

#ifdef SQLITE_ENABLE_DESERIALIZE
int sqlite3MemdbInit(void);
#endif

const char *sqlite3ErrStr(int);
int sqlite3ReadSchema(Parse *pParse);
CollSeq *sqlite3FindCollSeq(sqlite3*,u8 enc, const char*,int);
int sqlite3IsBinary(const CollSeq*);
CollSeq *sqlite3LocateCollSeq(Parse *pParse, const char*zName);
CollSeq *sqlite3ExprCollSeq(Parse *pParse, Expr *pExpr);
CollSeq *sqlite3ExprNNCollSeq(Parse *pParse, Expr *pExpr);
int sqlite3ExprCollSeqMatch(Parse*,Expr*,Expr*);
Expr *sqlite3ExprAddCollateToken(Parse *pParse, Expr*, const Token*, int);
Expr *sqlite3ExprAddCollateString(Parse*,Expr*,const char*);
Expr *sqlite3ExprSkipCollate(Expr*);

      if( pExpr->iTable<0 ){
        /* This only happens when coding check constraints */
        sqlite3TreeViewLine(pView, "COLUMN(%d)%s", pExpr->iColumn, zFlgs);
      }else{
        sqlite3TreeViewLine(pView, "{%d:%d}%s",
                             pExpr->iTable, pExpr->iColumn, zFlgs);
      }
      if( ExprHasProperty(pExpr, EP_FixedCol) ){
        sqlite3TreeViewExpr(pView, pExpr->pLeft, 0);
      }
      break;
    }
    case TK_INTEGER: {
      if( pExpr->flags & EP_IntValue ){
        sqlite3TreeViewLine(pView, "%d", pExpr->u.iValue);
      }else{
        sqlite3TreeViewLine(pView, "%s", pExpr->u.zToken);

** by pCCurHint.iTabCur, and an index is being used (which we will
** know because CCurHint.pIdx!=0) then transform the TK_COLUMN into
** an access of the index rather than the original table.
*/
static int codeCursorHintFixExpr(Walker *pWalker, Expr *pExpr){
  int rc = WRC_Continue;
  struct CCurHint *pHint = pWalker->u.pCCurHint;
  if( pExpr->op==TK_COLUMN && !ExprHasProperty(pExpr, EP_FixedCol) ){
    if( pExpr->iTable!=pHint->iTabCur ){
      Vdbe *v = pWalker->pParse->pVdbe;
      int reg = ++pWalker->pParse->nMem;   /* Register for column value */
      sqlite3ExprCodeGetColumnOfTable(
          v, pExpr->pTab, pExpr->iTable, pExpr->iColumn, reg
      );
      pExpr->op = TK_REGISTER;

  aff2 = sqlite3ExprAffinity(pExpr->pRight);
  if( aff1!=aff2
   && (!sqlite3IsNumericAffinity(aff1) || !sqlite3IsNumericAffinity(aff2))
  ){
    return 0;
  }
  pColl = sqlite3BinaryCompareCollSeq(pParse, pExpr->pLeft, pExpr->pRight);
  if( sqlite3IsBinary(pColl) ) return 1;
  return sqlite3ExprCollSeqMatch(pParse, pExpr->pLeft, pExpr->pRight);
}

/*
** Recursively walk the expressions of a SELECT statement and generate
** a bitmask indicating which tables are used in that expression
** tree.

/*
** These routines walk (recursively) an expression tree and generate
** a bitmask indicating which tables are used in that expression
** tree.
*/
Bitmask sqlite3WhereExprUsageNN(WhereMaskSet *pMaskSet, Expr *p){
  Bitmask mask;
  if( p->op==TK_COLUMN && !ExprHasProperty(p, EP_FixedCol) ){
    return sqlite3WhereGetMask(pMaskSet, p->iTable);
  }else if( ExprHasProperty(p, EP_TokenOnly|EP_Leaf) ){
    assert( p->op!=TK_IF_NULL_ROW );
    return 0;
  }
  mask = (p->op==TK_IF_NULL_ROW) ? sqlite3WhereGetMask(pMaskSet, p->iTable) : 0;
  if( p->pLeft ) mask |= sqlite3WhereExprUsageNN(pMaskSet, p->pLeft);