CXform

CXforml类作为所有transformation的基础类，其包含了pattern成员m_pexpr。主要是在exploration和implementation expression流程中使用，主要调用Transform函数。其还包含返回相关xforms的集合函数，比如PbsIndexJoinXforms等。

class CXform : public CRefCount {
private:	CExpression *m_pexpr; // pattern	
	        CXform(CXform &); // private copy ctor

public:	
	explicit CXform(CExpression *pexpr); virtual ~CXform(); 
	CExpression *PexprPattern() const { return m_pexpr; } // accessor
	static BOOL FEqualIds(const CHAR *szIdOne, const CHAR *szIdTwo); // equality function over xform ids
	
	virtual EXformId Exfid() const = 0; // ident accessors	
	virtual const CHAR *SzId() const = 0; // return a string for xform name
	// the following functions check xform type	
	virtual BOOL FSubstitution() const { return false; } // is xform substitution?	
	virtual BOOL FExploration() const { return false; } // is xform exploration?	
	virtual BOOL FImplementation() const { return false; } // is xform implementation?
    virtual BOOL FCompatible(CXform::EXformId) { return true; } // check compatibility with another xform
	// actual transformation
	virtual void Transform(CXformContext *pxfctxt, CXformResult *pxfres, CExpression *pexpr) const = 0;
	// compute xform promise for a given expression handle
	virtual EXformPromise Exfp(CExpressionHandle &exprhdl) const = 0;
	// return true if xform should be applied only once.
	// for expression of type CPatternTree, in deep trees, the number
	// of expressions generated for group expression can be significantly
	// large causing the Xform to be applied many times. This can lead to
	// significantly long planning time, so such Xform should only be applied once
	virtual BOOL IsApplyOnce();

	// returns a set containing all xforms related to index join
	// caller takes ownership of the returned set
	static CBitSet *PbsIndexJoinXforms(CMemoryPool *mp);
	// returns a set containing all xforms related to bitmap indexes
	// caller takes ownership of the returned set
	static CBitSet *PbsBitmapIndexXforms(CMemoryPool *mp);
	// returns a set containing all xforms related to heterogeneous indexes
	// caller takes ownership of the returned set
	static CBitSet *PbsHeterogeneousIndexXforms(CMemoryPool *mp);
	// returns a set containing all xforms that generate a plan with a hash join
	// caller takes ownership of the returned set
	static CBitSet *PbsHashJoinXforms(CMemoryPool *mp);
	// returns a set containing xforms to use only the join order as available
	// in the query
	static CBitSet *PbsJoinOrderInQueryXforms(CMemoryPool *mp);
	// returns a set containing xforms to use combination of greedy xforms
	// for join order
	static CBitSet *PbsJoinOrderOnGreedyXforms(CMemoryPool *mp);
	// returns a set containing xforms to use for exhaustive join order
	static CBitSet *PbsJoinOrderOnExhaustiveXforms(CMemoryPool *mp);
	// returns a set containing xforms to use for exhaustive2 join order
	static CBitSet *PbsJoinOrderOnExhaustive2Xforms(CMemoryPool *mp);
};	// class CXform

CXformExploration和CXformImplementation

CXformExploration类作为Base class for all explorations，其定义如下所示：

class CXformExploration : public CXform {
private: CXformExploration(const CXformExploration &); // private copy ctor
public:	
	explicit CXformExploration(CExpression *pexpr); // ctor	
	virtual ~CXformExploration(); // dtor
	
	virtual BOOL FExploration() const { return true; } // type of operator
	// is transformation a subquery unnesting (Subquery To Apply) xform?
	virtual BOOL FSubqueryUnnesting() const { return false; }
	// is transformation an Apply decorrelation (Apply To Join) xform?
	virtual BOOL FApplyDecorrelating() const { return false; }
	// do stats need to be computed before applying xform?
	virtual BOOL FNeedsStats() const { return false; }
	// conversion function
	static CXformExploration *Pxformexp(CXform *pxform) {
		return dynamic_cast<CXformExploration *>(pxform);
	}

};	// class CXformExploration

CXformImplementation类作为base class for all implementations，其定义如下所示：

class CXformImplementation : public CXform {
private: CXformImplementation(const CXformImplementation &); // private copy ctor
public:	
	explicit CXformImplementation(CExpression *); // ctor	
	virtual ~CXformImplementation(); // dtor	
	virtual BOOL FImplementation() const { // type of operator
		return true;
	}

};	// class CXformImplementation