Visitor

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Deprecated

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org.apache.derby.iapi.sql.compile
Interface Visitor

public interface Visitor

A visitor is an object that traverses the querytree and performs some action.

Method Summary

boolean skipChildren(Visitable node)
          Method that is called to indicate whether we should skip all nodes below this node for traversal.

boolean stopTraversal()
          Method that is called to see if query tree traversal should be stopped before visiting all nodes.

Visitable visit(Visitable node)
          This is the default visit operation on a QueryTreeNode.

Method Detail

visit

public Visitable visit(Visitable node)
                throws org.apache.derby.iapi.error.StandardException

This is the default visit operation on a QueryTreeNode. It just returns the node. This will typically suffice as the default visit operation for most visitors unless the visitor needs to count the number of nodes visited or something like that.

Visitors will overload this method by implementing a version with a signature that matches a specific type of node. For example, if I want to do something special with aggregate nodes, then that Visitor will implement a visit(AggregateNode node) method which does the aggregate specific processing.

stopTraversal

public boolean stopTraversal()

Method that is called to see if query tree traversal should be stopped before visiting all nodes. Useful for short circuiting traversal if we already know we are done.

skipChildren

public boolean skipChildren(Visitable node)

Method that is called to indicate whether we should skip all nodes below this node for traversal. Useful if we want to effectively ignore/prune all branches under a particular node.

Differs from stopTraversal() in that it only affects subtrees, rather than the entire traversal.