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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-4416) Handle comparison of two constants as a boolean constant
Date Wed, 21 Oct 2009 11:41:59 GMT
Handle comparison of two constants as a boolean constant
--------------------------------------------------------

                 Key: DERBY-4416
                 URL: https://issues.apache.org/jira/browse/DERBY-4416
             Project: Derby
          Issue Type: Improvement
          Components: SQL
    Affects Versions: 10.5.3.0
            Reporter: Knut Anders Hatlen
            Priority: Minor


In the lack of the boolean data type, Derby forces you to use expressions like 1=1 and 1<>1
to express true and false. Generated SQL statements also tend to use such expressions, and
so does Derby in its own meta-data queries.

Derby has many useful optimizations for boolean true/false. For instance, ProjectRestrictNode
and PredicateList are able to eliminate predicates, and in some cases the entire ProjectRestrictNode,
if the predicate contains constant true or false values. However, during parsing and compilation,
expressions like 1=1 are not rewritten to TRUE, and we don't get any benefit from the boolean
optimization code. This leads to more complex, and possibly less efficient, byte code being
generated for the statements.

Also, boolean constants are assigned a selectivity of 0.0 (false) or 1.0 (true), since they
will always match no rows when false and all rows when true. The expression 1=1 does however
get it's selectivity from the = operator, which means that it'll be 0.1. The same selectivity
is assigned to 1=0. Other operators have different selectivity, so 2<3 has the selectivity
0.33, even though the actual selectivity of the expression is the same as 1=1 and TRUE, namely
1.0.

This leads to oddities like the optimizer choosing a different plan when you change 2<3
to 1=1 in a WHERE clause. See http://mail-archives.apache.org/mod_mbox/db-derby-user/200909.mbox/%3c25531166.post@talk.nabble.com%3e
for an example of that.

If we could go through the query tree and replace occurrences of comparisons between constant
values with a boolean constant at bind time, such queries would end up with simpler byte code,
and the selectivity passed to the optimizer would be more accurate, possibly resulting in
a better plan being chosen.

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