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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-4416) Handle comparison of two constants as a boolean constant
Date Wed, 21 Oct 2009 12:10:59 GMT

     [ https://issues.apache.org/jira/browse/DERBY-4416?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Knut Anders Hatlen updated DERBY-4416:
--------------------------------------

    Attachment: replaceExpressions.diff

Attached is a patch (not for commit) that I've experimented with. It adds a visitor that replaces
(some) expressions that are guaranteed to evaluate to true or false with a boolean constant
node. This visitor is used on the WHERE clauses and HAVING clauses from SelectNode.normExpressions()
and on the ON clauses in JoinNode.normExpressions().

I've seen that for instance this statement

  SELECT * FROM T WHERE 1=1

is translated into a ProjectRestrictResultSet on top of a table scan without the patch, and
simply to a table scan with the patch.

I tested it with suites.All and derbyall, and saw three failures:

1) lang/outerjoin.sql: Changed query plans. Looks like the expected changes, in that some
ProjectRestrictResultSets are eliminated, but will have to check to be sure.

2) lang/specjPlans.sql: Changed query plans. These also look benign, but will have to check.

3) lang/logop.sql: Query that previously failed now succeeds and returns no rows. Expected
output is below, and judging by the comment in the test, it looks as if the empty result that
was actually supposed to be the expected one. The reason why it stops failing, is that (1=2)
is rewritten to false and PredicateList.restorePredicates() is therefore able eliminate the
other half of the AND node (2147483647 + 10 = 2) that causes integer overflow.

ij> -- ... and false and ... should get resolved to false
select x from s where 2147483647 + 10 = 2 and (1=2);
ERROR 22003: The resulting value is outside the range for the data type INTEGER.

Now:

ij> -- ... and false and ... should get resolved to false
select x from s where 2147483647 + 10 = 2 and (1=2);
X          
-----------

> 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
>         Attachments: replaceExpressions.diff
>
>
> 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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