Description
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
Attachments
Attachments
Issue Links
- blocks
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DERBY-4416 Handle comparison of two constants as a boolean constant
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- Closed
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