Interface Visitor
- All Known Subinterfaces:
ASTVisitor
- All Known Implementing Classes:
BaseTableNumbersVisitor
,CollectNodesVisitor
,ConditionalNode.ReplaceCaseOperandVisitor
,ConstantExpressionVisitor
,HasCorrelatedCRsVisitor
,HasNodeVisitor
,HasTableFunctionVisitor
,HasVariantValueNodeVisitor
,OffsetOrderVisitor
,ReferencedTablesVisitor
,RemapCRsVisitor
,ReplaceAggregatesWithCRVisitor
,ReplaceNodeVisitor
,ReplaceWindowFuncCallsWithCRVisitor
,SubstituteExpressionVisitor
,VerifyAggregateExpressionsVisitor
,VTIDeferModPolicy
public interface Visitor
A visitor is an object that traverses the querytree
and performs some action.
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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.This is the default visit operation on a QueryTreeNode.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 Details
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visit
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.
- 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).
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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.- Parameters:
node
- the top node of a sub-tree about to be visited- Returns:
true
ifnode
's children should be visited beforenode
,false
otherwise
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stopTraversal
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.- Returns:
- true/false
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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.
- Parameters:
node
- the node to process- Returns:
- true/false
- Throws:
StandardException
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