Clinton,
how can I get inconsistent data with my approach? I think we misunderstood each other ;-) My usecase was the
following (and I think it was also the usecase of the OP):
Book --< Author = A book can have multiple authors.
Now I want to select the first 100 books, sorted alpabetically, a book filled up with its authors. So I do
have the following resultMaps
<resultMap id="book" class="Book" groupBy="bookId">
<result property="bookId"/>
<result property="title"/>
<result property="authors" resultMap="author"/>
</resultMap>
<resultMap id="author" class="Author">
<result property="authorId"/>
<result property="bookId"/>
<result property="name"/>
</resultMap>
Here I don't see why the following select should lead to a inconsistent view of
records...
<select id="booksWithTheirAuthors" resultMap="book">
select * from book, author where book.bookId = author.bookId
and book.bookId in (select bookId from book order by title asc limit 100)
</select>
Of course you could do this in two separate selects without having any impact
on the results.
--- Original Nachricht ---
Absender: Clinton Begin
Datum: 03.09.2008 16:47
But Kai, using that approach, you will get an inconsistent view of the
records. Some child records will be missing.
the way that ORMs do this is with two queries...
select distinct P.ID from PERSON P, DEPARTMENT D .... LIMIT 100
select * from PERSON P, DEPARTMENT D ..... and P.ID in (23, 45, 63, .... 104)
iBATIS cannot do that automatically. It's a limitation of not
generating the SQL. You can achieve the same thing by writing both
queries yourself though.
Clinton
On Wed, Sep 3, 2008 at 8:31 AM, Kai Grabfelder <[EMAIL PROTECTED]> wrote:
Hi Reuben,
regarding your first problem: Have you tried to give a comma separated list
for the groupBy criteria? Just an idea, until now I was only grouping for
one element not multiple ones as it can get quite expensive to do n+1 joins
on several tables on the database level.
regarding your second problem: I've used subselects in such cases that limit
the number record returned. This may be not the fastest solution for all
usescases but it works. In your case it could look like this:
select * from book_table, other_join_tables where book_table.id =
other_join_tables.book_id and
book_table.id in (select id from book_table order by sort_crit desc limit
100 offset 10)
Regards
Kai
--- Original Nachricht ---
Absender: Clinton Begin
Datum: 03.09.2008 16:20
The second problem is a limitation that we cannot do anything about,
which makes the rest of the conversation somewhat FYI only.
The first problem does sound like a bug, but strangely I have unit
tests confirming that this works. I'll try writing a few more to see
if I can reproduce the problem. It very well might be the combination
of keys being used in the groupBy attribute or something, but I'll
check it out to see.
Clinton
On Wed, Sep 3, 2008 at 7:32 AM, Reuben Firmin <[EMAIL PROTECTED]>
wrote:
Anybody have any feedback on this?
Thanks
Reuben
---------- Forwarded message ----------
From: Reuben Firmin <[EMAIL PROTECTED]>
Date: Tue, Sep 2, 2008 at 11:26 AM
Subject: GroupBy issues (multiple child lists, Postgres limit/offset)
To: [email protected]
We are trying to resolve some N+1 query situations in our application,
and
are finding a couple of features of our appliation that seem to limit our
ability to use the "groupBy" solution. I'm wondering if there are aspects
of
the issues we aren't seeing.
The problems are these:
1. In places where we have an object structure that has a parent with
multiple child lists, it appears that we can't use groupBy to get all of
the
results with one query. For example,
class Book {
...
List<Author> authors;
List<Comment> comments;
List<Subject> subjects;
...
For this type of situation, it seems like our choices are to (a) use
groupBy
for one of the child lists, and selects in the resultMap for the other
children (doesn't completely solve N+1 problem, just reduces it), or (b)
using a cross-product join of all tables and a custom RowHandler to
manage
it all with one query.
2. We are using Postgresql, and taking advantage of the "limit" and
"offset"
keywords to help implement paging of the results we display - the "limit"
and "offset" values correspond to the "Results (offset) - (offset +
limit)
of (n)" message we can display to users. It seems that these aren't going
to
be compatible with a "groupBy" approach since "limit" and "offset" work
at
the resultSet level, and "groupBy" works by having a resultSet that's a
cross product of at least a couple of tables. That is, we want to rely on
the limit and offset ability at the database level (makes queries and
resultset handling simpler), but the values refer to domain entities and
not
resultset rows. We can use the keywords if we aren't worried about N+1
selects, but the values will lose their domain entity meaning if we do
cross
product queries with groupBy. Is there any way that people have found
around
this?
Thanks for any advice,
Reuben
