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From "wangwenli (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-6358) WHERE clause should be evaluated after the joins in the FROM clause
Date Thu, 15 Jun 2017 08:13:00 GMT

    [ https://issues.apache.org/jira/browse/DERBY-6358?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16050135#comment-16050135
] 

wangwenli commented on DERBY-6358:
----------------------------------


the reporter says:
{quote}
See part 2 of the SQL Standard, section 7.4 (<table expression>), general rule 1
{quote}
i copied the rule here:
{quote}
If all optional clauses are omitted, then the result of the <table expression> is the
same as
the result of the <from clause>. Otherwise, each specified clause is applied to the
result of
the previously specified clause and the result of the <table expression> is the result
of the
application of the last specified clause.
{quote}

i cann't see putting where clause before join violate the rule, any detail explanation?

> 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: 10.2.2.1, 10.10.2.0, 10.11.1.1
>            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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