[jira] [Created] (DERBY-6358) WHERE clause should be evaluated after the joins in the FROM clause

["Rick Hillegas (JIRA)" <ji...@apache.org>]

Rick Hillegas created DERBY-6358:
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             Summary: WHERE clause should be evaluated after the joins in the FROM clause
                 Key: DERBY-6358
                 URL: https://issues.apache.org/jira/browse/DERBY-6358
             Project: Derby
          Issue Type: Bug
          Components: SQL
    Affects Versions: 10.11.0.0
            Reporter: Rick Hillegas


The WHERE clause is supposed to be evaluated after the inner and outer joins specified in the FROM clause. See part 2 of the SQL Standard, section 7.4 (<table expression>), general rule 1. However, it appears that Derby flattens the inner joins into a cartesian product and mixes their ON clauses into the WHERE clause. As a result, WHERE clause fragments can be evaluated before the ON clauses. The following script shows this problem:

connect 'jdbc:derby:memory:db;create=true';

create table t1( a varchar( 10 ) );
create table t2( a varchar( 10 ) );

insert into t1( a ) values ( 'horse' ), ( 'apple' ), ( 'star' ), ( '6' );
insert into t2( a ) values ( '6' );

-- ok if the cast is performed in the select list
select cast( t1.a as int )
from t1 inner join t2 on t1.a = t2.a;

-- should succeed.
-- but we see a casting error because the WHERE clause is evaluated before the ON clause
select *
from t1 inner join t2 on t1.a = t2.a
where cast( t1.a as int ) > 5;

Fixing this bug may result in serious performance degradation for many queries. A release note will be needed to tell users how to re-write their queries in order to get the old performance. For instance, the user may need to flatten the inner joins themselves, rewriting the query as a cartesian product with a WHERE clause.



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