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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