First, an _expression_ "matching" facility. The first cut can be restricted to looking at two expressions and detecting if they are the "same"-- by same, I mean, they will evaluate to the same value at runtime. Thus the _expression_ "f()" is not the same as "f()". (I don't see a way in the derby docs to declare a function as "deterministic"-- is that correct?)
Second, the group by binding/validation code will need to lose assumptions about grouping expressions being columns. We'll also have to do a little query rewrite (I'm guessing) to replace references to the grouping expressions in the having/select clause.
When we have this functionality ofcourse, as you suggest in your email, we'll look into allowing subexpressions and such.
As far as the first step goes, I plan on adding a function to the base class (ValueNode).
protected boolean isEquivalent(ValueNode other)
{
return false;
}
and having each subclass override this method and ofcourse do the right thing.
I'll have to spend time looking at the ValueNode hierarchy and see what the correct behavior for "isEquivalent" is going to be for each class.
Just wanted to swing this proposal by people more familiar with the code. If you see anything wrong in this proposal, let me know. I'll probably start on it in a day or two.
cheers
Manish
On 5/23/06, Satheesh Bandaram <[EMAIL PROTECTED]> wrote:
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?
SatheeshOn 5/20/06, Manish Khettry <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> wrote:No, we don't go quite that far. Because most dbs support group by expressions, we haven't needed to implement this.
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 < [EMAIL PROTECTED]> 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:[email protected]]
Sent: Wednesday, May 17, 2006 1:14 PM
To: [email protected]
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)
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