Apache Derby Database Engine and Embedded JDBC Driver 10.17.1.0 · org.apache.derby.impl.sql.compile.ConstantExpressionVisitor

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.

Access	Constructor and Description
pack-priv	ConstantExpressionVisitor()

Method Summary

Modifier and Type	Method and Description
public boolean	Returns: `false`, since the entire tree should be visited skipChildren(Visitable the node to process node) Implements org.apache.derby.iapi.sql.compile.Visitor.skipChildren. Method that is called to indicate whether we should skip all nodes below this node for traversal.
public boolean	Returns: `false`, since the entire tree should be visited stopTraversal() Implements org.apache.derby.iapi.sql.compile.Visitor.stopTraversal. Method that is called to see if query tree traversal should be stopped before visiting all nodes.
public Visitable	visit(Visitable the node to process node) Implements org.apache.derby.iapi.sql.compile.Visitor.visit. Visit the node and call `evaluateConstantExpressions()` if it is a `ValueNode`.
public boolean	Returns: `true`, since the tree should be walked bottom-up visitChildrenFirst(Visitable the top node of a sub-tree about to be visited node) Implements org.apache.derby.iapi.sql.compile.Visitor.visitChildrenFirst. Method that is called to see if `visit()` should be called on the children of `node` before it is called on `node` itself.

Inherited from java.lang.Object:: clone equals finalize getClass hashCode notify notifyAll toString wait wait wait

Constructor Detail

ConstantExpressionVisitor	back to summary
pack-priv ConstantExpressionVisitor()

Method Detail

skipChildren back to summary

skipChildren	back to summary
public boolean skipChildren(Visitable node) Implements org.apache.derby.iapi.sql.compile.Visitor.skipChildren. Doc from org.apache.derby.iapi.sql.compile.Visitor.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:Visitable the node to process Returns:boolean `false`, since the entire tree should be visited

public boolean skipChildren(Visitable node)

Implements org.apache.derby.iapi.sql.compile.Visitor.skipChildren.

Doc from org.apache.derby.iapi.sql.compile.Visitor.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:Visitable: the node to process

Returns:boolean

false, since the entire tree should be visited

stopTraversal back to summary

stopTraversal	back to summary
public boolean stopTraversal() Implements org.apache.derby.iapi.sql.compile.Visitor.stopTraversal. Doc from org.apache.derby.iapi.sql.compile.Visitor.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:boolean `false`, since the entire tree should be visited

public boolean stopTraversal()

Implements org.apache.derby.iapi.sql.compile.Visitor.stopTraversal.

Doc from org.apache.derby.iapi.sql.compile.Visitor.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:boolean: false, since the entire tree should be visited

visit back to summary

visit	back to summary
public Visitable visit(Visitable node) throws StandardException Implements org.apache.derby.iapi.sql.compile.Visitor.visit. Visit the node and call `evaluateConstantExpressions()` if it is a `ValueNode`. Parameters node:Visitable Doc from org.apache.derby.iapi.sql.compile.Visitor.visit. the node to process Returns:Visitable Doc from org.apache.derby.iapi.sql.compile.Visitor.visit. 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. Exceptions StandardException: Doc from org.apache.derby.iapi.sql.compile.Visitor.visit. 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()`

public Visitable visit(Visitable node) throws StandardException

Implements org.apache.derby.iapi.sql.compile.Visitor.visit.

Visit the node and call evaluateConstantExpressions() if it is a ValueNode.

Parameters

node:Visitable

Doc from org.apache.derby.iapi.sql.compile.Visitor.visit.

the node to process

Returns:Visitable

Doc from org.apache.derby.iapi.sql.compile.Visitor.visit.

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.

Exceptions

StandardException:

Doc from org.apache.derby.iapi.sql.compile.Visitor.visit.

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).

visitChildrenFirst back to summary

visitChildrenFirst	back to summary
public boolean visitChildrenFirst(Visitable node) Implements org.apache.derby.iapi.sql.compile.Visitor.visitChildrenFirst. Doc from org.apache.derby.iapi.sql.compile.Visitor.visitChildrenFirst. 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. Parameters node:Visitable the top node of a sub-tree about to be visited Returns:boolean `true`, since the tree should be walked bottom-up

public boolean visitChildrenFirst(Visitable node)

Implements org.apache.derby.iapi.sql.compile.Visitor.visitChildrenFirst.

Doc from org.apache.derby.iapi.sql.compile.Visitor.visitChildrenFirst.

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.

Parameters

node:Visitable: the top node of a sub-tree about to be visited

Returns:boolean

true, since the tree should be walked bottom-up

pack-priv Class ConstantExpressionVisitor

Constructor Summary

Method Summary

Constructor Detail

Method Detail