Package org.apache.derby.impl.sql.compile

Class ConstantExpressionVisitor

  • All Implemented Interfaces:
    Visitor
    class ConstantExpressionVisitor
extends java.lang.Object
implements Visitor

    This visitor replaces a ValueNode with a node representing a constant value, if the ValueNode is known to always evaluate to the same value. It may for instance replace a sub-tree representing 1=1 with a constant TRUE.

    The actual evaluation of the ValueNodes is performed by invoking ValueNode.evaluateConstantExpressions() on every ValueNode in the query tree.

    In contrast to most other visitors, this visitor walks the tree bottom-up. Top-down processing of the tree would only evaluate constant expressions at the leaf level, so for instance (1=1)=(1=2) would only be simplified to TRUE=FALSE. With bottom-up processing, the top-level = node will be processed after the leaves, and it sees the intermediate tree TRUE=FALSE which it is able to transform into the even simpler tree FALSE.

    • Method Summary

      All Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      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)
      Visit the node and call evaluateConstantExpressions() if it is a ValueNode.
      boolean visitChildrenFirst​(Visitable node)
      Method that is called to see if visit() should be called on the children of node before it is called on node itself.

      • Methods inherited from class java.lang.Object

        clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

    • Constructor Detail

      • ConstantExpressionVisitor

        ConstantExpressionVisitor()

    • Method Detail

      • visit

        public Visitable visit​(Visitable node)
                throws StandardException
        Visit the node and call evaluateConstantExpressions() if it is a ValueNode.
        Specified by:
        visit in interface Visitor
        Parameters:
        node - the node to process
        Returns:
        a query tree node. Often times this is the same node that was passed in, but Visitors that replace nodes with other nodes will use this to return the new replacement node.
        Throws:
        StandardException - may be throw an error as needed by the visitor (i.e. may be a normal error if a particular node is found, e.g. if checking a group by, we don't expect to find any ColumnReferences that aren't under an AggregateNode -- the easiest thing to do is just throw an error when we find the questionable node).
        See Also:
        ValueNode.evaluateConstantExpressions()

      • 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.
        Specified by:
        stopTraversal in interface Visitor
        Returns:
        false, since the entire tree should be visited

      • 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.

        Specified by:
        skipChildren in interface Visitor
        Parameters:
        node - the node to process
        Returns:
        false, since the entire tree should be visited

      • visitChildrenFirst

        public boolean visitChildrenFirst​(Visitable node)
        Method that is called to see if visit() should be called on the children of node before it is called on node itself. If this method always returns true, the visitor will walk the tree bottom-up. If it always returns false, the tree is visited top-down.
        Specified by:
        visitChildrenFirst in interface Visitor
        Parameters:
        node - the top node of a sub-tree about to be visited
        Returns:
        true, since the tree should be walked bottom-up