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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-4421) Allow Visitors to process the nodes bottom-up
Date Fri, 23 Oct 2009 11:19:59 GMT
Allow Visitors to process the nodes bottom-up
---------------------------------------------

                 Key: DERBY-4421
                 URL: https://issues.apache.org/jira/browse/DERBY-4421
             Project: Derby
          Issue Type: Improvement
          Components: SQL
    Affects Versions: 10.6.0.0
            Reporter: Knut Anders Hatlen
            Priority: Minor


Currently, QueryTreeNode.accept() walks the tree top-down, always calling
visit() on the parent before it calls visit() on the children. Although this
is fine in most cases, there are use cases where visiting the nodes
bottom-up would be better. One example is mentioned in DERBY-4416. The
visitor posted there looks for binary comparison operators and checks
whether both operands are constants. If they are, the operator is replaced
with a boolean constant.

Take this expression as an example: (1<2)=(2>1)

The query tree looks like this:

       =
     /   \
    /     \
   <       >
  / \     / \
 /   \   /   \
1     2 2     1

If we walk the tree top-down with the said visitor, the = node doesn't have
constant operands when it's visited. The < and > operators do have constant
operands, and they're both replaced with constant TRUE. This means the
expression (1<2)=(2>1) is rewritten to TRUE=TRUE, and that's how far the
transformation goes.

If the tree had been processed bottom-up, we would start with the < and >
operators, and again replace them with TRUE. The query tree would therefore
have been transformed to this intermediate form when the = operator was
visited:

       =
     /   \
    /     \
  TRUE   TRUE

This is the same as the end result when visiting top-down, but now the =
operator hasn't been visited yet. Since both the operands of the = operator
are constants, the visitor will perform yet another transformation so the
tree is simplified further and ends up as:

    TRUE

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