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https://issues.apache.org/jira/browse/DERBY-4631?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13185764#comment-13185764
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Mike Matrigali commented on DERBY-4631:
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I am concerned by the performance characteristics of you proposed solution #2.
Could you go into more detail on what it involves and what
set of queries it will affect. I am most interested in what happens at
execution time for a large join. I think my most basic question is in a
1 to 1 million row join will you be adding 1 million new function calls for the
coalesce, or is this somehow optimized after bind? I am assuming
that the reason the code does not currently do an explicit coalesce already is
an optimization, where it was assumed the implementation
behavior would match the external behavior that the spec is describing.
Solution 1 seems safer to me, and if implemented we should log the SQL syntax
problems you have uncovered in a separate JIRA.
Also can you explicitly give some queries that currently work today that will
not after your change, so that the compatibility impact of your
solutions can be understood. Do those queries return correct results? Once
you list a few of these queries maybe we can try them on other
platforms to verify that they are incorrect SQL.
> Wrong join column returned by right outer join with NATURAL or USING and
> territory-based collation
> --------------------------------------------------------------------------------------------------
>
> Key: DERBY-4631
> URL: https://issues.apache.org/jira/browse/DERBY-4631
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.6.1.0
> Reporter: Knut Anders Hatlen
> Assignee: Mamta A. Satoor
> Labels: derby_triage10_8
>
> SQL:2003 says that the join columns in a natural join or in a named
> columns join should be added to the select list by coalescing the
> column from the left table with the column from the right table.
> Section 7.7, <joined table>, syntax rules:
> > 1) Let TR1 be the first <table reference>, and let TR2 be the <table
> > reference> or <table factor> that is the second operand of the
> > <joined table>. Let RT1 and RT2 be the row types of TR1 and TR2,
> > respectively. Let TA and TB be the range variables of TR1 and TR2,
> > respectively. (...)
> and
> > 7) If NATURAL is specified or if a <join specification> immediately
> > containing a <named columns join> is specified, then:
> (...)
> > d) If there is at least one corresponding join column, then let SLCC
> > be a <select list> of <derived column>s of the form
> >
> > COALESCE ( TA.C, TB.C ) AS C
> >
> > for every column C that is a corresponding join column, taken in
> > order of their ordinal positions in RT1.
> For a right outer join, Derby doesn't use COALESCE(TA.C, TB.C), but
> rather just TB.C (the column in the right table) directly.
> This is in most cases OK, because COALESCE(TA.C, TB.C) = TB.C is an
> invariant in a right outer join. (Because TA.C is either NULL or equal
> to TB.C.)
> However, in a database with territory-based collation, equality
> between two values does not mean they are identical, especially now
> that the strength of the collator can be specified (DERBY-1748).
> Take for instance this join:
> ij> connect
> 'jdbc:derby:testdb;create=true;territory=en_US;collation=TERRITORY_BASED:SECONDARY';
> ij> create table big(x varchar(5));
> 0 rows inserted/updated/deleted
> ij> insert into big values 'A','B','C';
> 3 rows inserted/updated/deleted
> ij> create table small(x varchar(5));
> 0 rows inserted/updated/deleted
> ij> insert into small values 'b','c','d';
> 3 rows inserted/updated/deleted
> ij> select x, t1.x, t2.x, coalesce(t1.x, t2.x) from small t1 natural right
> outer join big t2;
> X |X |X |4
> -----------------------
> A |NULL |A |A
> B |b |B |b
> C |c |C |c
> 3 rows selected
> I believe that the expected result from the above query is that the
> first column should have the same values as the last column. That is,
> the first column should contain {'A', 'b', 'c'}, not {'A', 'B', 'C'}.
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