Class ConstantExpressionVisitor
- java.lang.Object
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- org.apache.derby.impl.sql.compile.ConstantExpressionVisitor
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- 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 theValueNode
is known to always evaluate to the same value. It may for instance replace a sub-tree representing1=1
with a constantTRUE
.The actual evaluation of the
ValueNode
s is performed by invokingValueNode.evaluateConstantExpressions()
on everyValueNode
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 toTRUE=FALSE
. With bottom-up processing, the top-level = node will be processed after the leaves, and it sees the intermediate treeTRUE=FALSE
which it is able to transform into the even simpler treeFALSE
.
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Constructor Summary
Constructors Constructor Description ConstantExpressionVisitor()
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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 callevaluateConstantExpressions()
if it is aValueNode
.boolean
visitChildrenFirst(Visitable node)
Method that is called to see ifvisit()
should be called on the children ofnode
before it is called onnode
itself.
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Method Detail
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visit
public Visitable visit(Visitable node) throws StandardException
Visit the node and callevaluateConstantExpressions()
if it is aValueNode
.- Specified by:
visit
in interfaceVisitor
- 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()
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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 interfaceVisitor
- Returns:
false
, since the entire tree should be visited
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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 interfaceVisitor
- Parameters:
node
- the node to process- Returns:
false
, since the entire tree should be visited
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visitChildrenFirst
public boolean visitChildrenFirst(Visitable node)
Method that is called to see ifvisit()
should be called on the children ofnode
before it is called onnode
itself. If this method always returnstrue
, the visitor will walk the tree bottom-up. If it always returnsfalse
, the tree is visited top-down.- Specified by:
visitChildrenFirst
in interfaceVisitor
- Parameters:
node
- the top node of a sub-tree about to be visited- Returns:
true
, since the tree should be walked bottom-up
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