Class ConstantExpressionVisitor
- All Implemented Interfaces:
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 ValueNode
s 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
.
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Constructor Summary
Constructors -
Method Summary
Modifier and TypeMethodDescriptionboolean
skipChildren
(Visitable node) Method that is called to indicate whether we should skip all nodes below this node for traversal.boolean
Method that is called to see if query tree traversal should be stopped before visiting all nodes.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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Constructor Details
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ConstantExpressionVisitor
ConstantExpressionVisitor()
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Method Details
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visit
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:
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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
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
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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