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https://issues.apache.org/jira/browse/OPENJPA-759?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13422828#comment-13422828
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Matt Shen commented on OPENJPA-759:
-----------------------------------
I like this patch.
It may not affect performance. But It will limit the size of this result list
from DB2 to JVM, when client passes a very large page number, which used to
generate a SQL like this:
select * from t1 FETCH FIRST 1000000 ROWS ONLY
correct me if I worry about "FETCH FIRST 1000000" unnecessarily.
> Query pagination on DB2 - an alternative way
> --------------------------------------------
>
> Key: OPENJPA-759
> URL: https://issues.apache.org/jira/browse/OPENJPA-759
> Project: OpenJPA
> Issue Type: Improvement
> Components: sql
> Affects Versions: 2.0.0-M2
> Environment: DB2 UDB
> Reporter: Milosz Tylenda
> Priority: Minor
> Attachments: DB2Dictionary.java, OPENJPA-759.patch, timer.zip
>
>
> The attached patch provides an alternative way of doing paging in DB2.
> I made it hoping that it would improve the performance of paging queries
> but it didn't, at least not when the database and OpenJPA are co-located.
> There is probably no point in committing this patch unless someone
> proves it gives any advantages.
> I am however presenting it here, maybe some users will find it useful
> (it would be nice if someone could check timings with remote database).
> Attached are:
> - a patch which modifies DB2Dictionary and also adds a few tests to
> TestQueryPagination.
> - a standalone modified DB2Dictionary class.
> - a timer program which was supposed to prove the superior performance but
> failed :)
> Some characteristics:
> - The idea is to limit the result set returned by SQL query instead of
> skipping rows
> when traversing the result set in OpenJPA. A similar approach exists
> in OracleDictionary.
> - If setMaxResulsts and setFirstResult were called on Query, the SQL query
> [QUERY] is modified as follows:
> SELECT * FROM (
> SELECT rr.*, ROW_NUMBER() OVER(ORDER BY ORDER OF rr) AS rn FROM (
> [QUERY]
> FETCH FIRST [m] ROWS ONLY
> ) AS rr
> ) AS r WHERE rn > [n] ORDER BY rn
> - The modified SQL query adds one column to the end of column list in the
> result set. Luckily, I couldn't find any side effects of doing this.
> - If only setMaxResults was called on Query, only FETCH FIRST [m] ROWS ONLY
> is appended to SQL query - this is how it works currently.
> - The new way of paging will be used only if the database is a UDB 8.1 or
> later
> because of ORDER OF construct and FETCH FIRST [m] ROWS ONLY in a subselect.
> Maybe some other DB2 flavours could also handle it but I have no access.
> - User can fall back to the old behaviour by setting supportsSelectStartIndex
> Dictionary property to false.
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