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From "Satheesh Bandaram" <banda...@gmail.com>
Subject Re: [jira] Commented: (DERBY-883) Enhance GROUP BY clause to support expressions instead of just column references.
Date Wed, 24 May 2006 00:29:03 GMT
Manish, good research. I think you are on the right track. Derby already
rewrites GROUP BY clause into select subquery so that HAVING clause can be
converted into a WHERE clause on top of select subquery, with aggregate
references converted into simple columnReferences. Search for groupByList in
sqlgrammar.jj. While this rewrite makes implementation easier without any
performance penality, it does confuse a few query resolutions. See
DERBY-280. (http://issues.apache.org/jira/browse/DERBY-280)

So, I think both the approaches below are essentially same thing, because of
current Derby rewrite. This rewrite could make implementing this feature
easier, like you said. Most of the work seems to be in the compile phase.

To start with, you could implement simple expression matching, which only
finds exact matches:

Select day(ts), count(*) from timeStampTab group by day(ts) having day(ts) =
1

Then it can be expanded to support proper sub-expressions.

Select (a+b)+c, count(*) from intTab group by a+b

Note that DB2 and Oracle don't seem to have fancy expression matching,
rejecting this:

Select b+a, count(*) from intTab group by a+b

so, Derby expression matching can be simple matching algorithm, to start
with. Good thing with this new feature is that it can be expanded
incrementally, without breaking existing or intermediate stage queries.

Does that help?

Satheesh

On 5/20/06, Manish Khettry <manish.khettry@gmail.com> wrote:
>
> Thanks Alex and Satheesh, that does clarify things quite a bit.
>
> I spent some time looking at the GroupedAggregateResultSet and I have the
> feeling that atleast on the execution side of things not much will have to
> change. A grouped aggregate RS already sits on top of a ProjectRestrictRS
> and if we can have the prRS evaluate the expressions we are grouping on, the
> existing logic should work without any (or perhaps minimal) changes. It is
> on the compilation side of things that this is, err rather hairy :) Someone
> should correct me if I'm missing something here or simplifying things.
>
> The other approach is to rewrite the query to use a select subquery like
> Satheesh suggested. Perhaps this is a better way to go. Any thoughts on
> which approach is better?
>
>
>
> On 5/18/06, Alex Miller <amiller@metamatrix.com> wrote:
> >
> >  No, we don't go quite that far.  Because most dbs support group by
> > expressions, we haven't needed to implement this.
> >
> >  ------------------------------
> > *From:* Satheesh Bandaram [mailto:bandaram@gmail.com]
> > *Sent:* Thursday, May 18, 2006 2:16 PM
> >
> > *To:* derby-dev@db.apache.org
> > *Subject:* Re: [jira] Commented: (DERBY-883) Enhance GROUP BY clause to
> > support expressions instead of just column references.
> >
> > Thanks Alex, for your message and the blogs... It was an interesting
> > reading. Wonder if MetaMatrix rewrites queries with GROUP BY expressions for
> > Derby into SELECT subqueries... something like:
> >
> > select year(hiredate), month(hiredate) as month, count(*)  from
> > employees
> > group by year(hiredate), month(hiredate);
> >
> > INTO something like:
> >
> > select years, months, count(*) from (select year(hiredate),
> > month(hiredate) from employees) emp (years,months) group by years, months;
> >
> > This could be used as a workaround until this functionality is added to
> > Derby.
> >
> > Satheesh
> >
> > On 5/18/06, Alex Miller < amiller@metamatrix.com> wrote:
> > >
> > > I blogged about GROUP BY expression support across dbs a while back
> > > (http://tech.puredanger.com/2005/03/02/fun-with-expressions-in-a-group-b
> > > y/ and
> > > http://tech.puredanger.com/2005/04/01/update-on-expressions-in-group-by/
> > >
> > > ).  Pretty much all the major vendors support expressions in a GROUP
> > > BY.
> > > I implemented this functionality a while back for the MetaMatrix query
> > >
> > > engine and blogged about using MetaMatrix Query to "add" this
> > > functionality over Derby in
> > > http://devcentral.metamatrix.com/blog/alex/2006/02/28/Enhancing-Apache-D
> > >
> > > erby-with-MetaMatrix.
> > >
> > > Positional parameters don't make as much sense for GROUP BY as they do
> > > for ORDER BY.  Logically, you're executing the clauses in the order
> > > FROM-WHERE-GROUP BY-HAVING-SELECT-ORDER BY.  The SELECT clause is the
> > > point at which the output columns of the query are effectively named
> > > and
> > > ordered.  Positional parameters make sense in ORDER BY because they
> > > refer to the output columns of the previous phase, which are defined
> > > as
> > > part of the query, but may have no well-defined name to refer to them
> > > with.
> > >
> > > In the case of GROUP BY, you would be referring backwards from the
> > > SELECT clause to the GROUP BY clause, so that seems kind of
> > > goofy.  The
> > > only reason to do this would be to avoid referencing a complex unnamed
> > > expression.  This is, of course, exactly the sort of thing that db
> > > vendors bend the rules about to make SQL more usable.  In fact, you
> > > can
> > > use GROUP BY positional parameters in MySQL and Postgres but not in
> > > any
> > > major commercial db that I've tried (Oracle, DB2, SQL Server, Sybase).
> > >
> > > Alex Miller
> > > Chief Architect
> > > MetaMatrix
> > >
> > > -----Original Message-----
> > > From: Satheesh Bandaram (JIRA) [mailto:derby-dev@db.apache.org]
> > > Sent: Wednesday, May 17, 2006 1:14 PM
> > > To: derby-dev@db.apache.org
> > > Subject: [jira] Commented: (DERBY-883) Enhance GROUP BY clause to
> > > support expressions instead of just column references.
> > >
> > >     [
> > > http://issues.apache.org/jira/browse/DERBY-883?page=comments#action_1241
> > >
> > > 2227 ]
> > >
> > > Satheesh Bandaram commented on DERBY-883:
> > > -----------------------------------------
> > >
> > > Thanks for making progress on this important new functionality,
> > > Manish.
> > >
> > > > 1. In terms of syntax, do we allow expressions in the group by list
> > > or
> > > positional parameters, or both?
> > > >
> > > > select tomonth(creationdt), toyear(creationdt), count(*) from bugs
> > > > group by 1, 2;
> > >
> > > I have seen positional parameters for ORDER BY expressions, not
> > > typically used in GROUP BY. Looking at both DB2 and Oracle
> > > documentation, it seems neither support positional parameters.
> > >
> > > > An implementation question on this note-- does the language code
> > > have
> > > > a way of looking at two expressions (ValueNode?) and checking to see
> > > > if they are equivalent? We'll need some way of doing this to match
> > > an
> > > > expression in the group by list to an expression in the select list
> > > right?
> > >
> > > Correct. Don't think there is any existing expression matching to
> > > compare two expressions. DB2 docs discuss how group by expressions are
> > > matched in SQL reference manual. (Page 484:
> > > ftp://ftp.software.ibm.com/ps/products/db2/info/vr82/pdf/en_US/db2s1e81
> > > .
> > > pdf)
> > >
> > > > 2. I assume that an expression in a group by list must appear in the
> > > > select list without aggregation right? Is this an error?
> > > >
> > > > select x+1, x+2, sum(y)
> > > > from test
> > > > group by x
> > >
> > > NO... This is a valid query. See the reference I provided above.
> > >
> > > > 3. What do we do with duplicates? i.e.
> > > >
> > > > select x+1, x+1, sum(y)
> > > > from test
> > > > group by x+1, x+1;
> > > >
> > > > Is this an error? The current implementation throws an error if the
> > > > same column occurs more than once in the group by list.
> > >
> > > I am not sure why Derby currently considers this an error... Looking
> > > at
> > > the code, it seems it may be looking for ambiguous column references
> > > (like 'x' being part of two different tables in from_list), which
> > > makes
> > > sense, but not sure why duplicate references should be prevented.
> > >
> > > > Is there a standard somewhere which I should consult before trying
> > > to
> > > nail down the functionality?
> > >
> > > Unfortunately, NO.... SQL 2003 seems to allow only column references
> > > in
> > > GROUP BY clause. But both DB2 and Oracle allow expressions in GROUP BY
> > >
> > > list and likely allowed by other database vendors too. You could use
> > > either DB2 or Oracle docs to understand how this functionality is
> > > defined there. Much easier to read these docs than confusing SQL 2003
> > > spec.
> > >
> > >
> > > > Enhance GROUP BY clause to support expressions instead of just
> > > column
> > > references.
> > > >
> > > ----------------------------------------------------------------------
> > > > -----------
> > > >
> > > >          Key: DERBY-883
> > > >          URL: http://issues.apache.org/jira/browse/DERBY-883
> > > >      Project: Derby
> > > >         Type: New Feature
> > >
> > > >   Components: SQL
> > > >     Versions: 10.1.2.1
> > > >  Environment: JDK 1.5.0_05
> > > >     Reporter: Lluis Turro
> > > >     Assignee: Manish Khettry
> > >
> > > >
> > > > This select would return an error syntax on finding "(" after month
> > > if
> > > group by clause:
> > > > select idissue, month(creation), year(creation), count(distinct
> > > > idissue) where
> > > >   ....
> > > > group by idissue, month(creation), year(creation)
> > >
> > > --
> > > This message is automatically generated by JIRA.
> > > -
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> > >
> > >
> >
>

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