Example #1
0
//---------------------------------------------------------------------------
//	@function:
//		CJoinOrderDP::DCost
//
//	@doc:
//		Primitive costing of join expressions;
//		cost of a join expression is the summation of the costs of its
//		children plus its local cost;
//		cost of a leaf expression is the estimated number of rows
//
//---------------------------------------------------------------------------
CDouble
CJoinOrderDP::DCost
	(
	CExpression *pexpr
	)
{
	GPOS_CHECK_STACK_SIZE;
	GPOS_ASSERT(NULL != pexpr);

	CDouble *pd = m_phmexprcost->Find(pexpr);
	if (NULL != pd)
	{
		// stop recursion if cost was already cashed
		return *pd;
	}

	CDouble dCost(0.0);
	const ULONG arity = pexpr->Arity();
	if (0 == arity)
	{
		// leaf operator, use its estimated number of rows as cost
		dCost = CDouble(pexpr->Pstats()->Rows());
	}
	else
	{
		// inner join operator, sum-up cost of its children
		DOUBLE rgdRows[2] = {0.0,  0.0};
		for (ULONG ul = 0; ul < arity - 1; ul++)
		{
			CExpression *pexprChild = (*pexpr)[ul];

			// call function recursively to find child cost
			dCost = dCost + DCost(pexprChild);
			DeriveStats(pexprChild);
			rgdRows[ul] = pexprChild->Pstats()->Rows().Get();
		}

		// add inner join local cost
		dCost = dCost + (rgdRows[0] + rgdRows[1]);
	}

	return dCost;
}