Pipelined XPath Query Based on Cost Optimization

Table 3

Symbols and their meanings.

Symbol	Meaning
	The region encoding of an XML node u. u is generally represented by the ID value.
	The relation index of an XML node u (see Definition 1)
	The sequence of pipelined query primitives corresponding to the XPath expression Ep. If Ep is a query step, it represents a query primitive.
	The number of all nodes with tag name in XML document D.
	The number of all nodes in the subtrees whose roots are nodes with tag name in XML document D.
	The number of nodes with relation and tag name contained in the subtrees whose roots are nodes with tag name in XML document D. That is, the number of nodes satisfying the relation condition in XML document D.
	Containing-mean (see Definition 3)
	Filtering-rate (see Definition 4)