Query pagination on DB2 - an alternative way
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Key: OPENJPA-759
URL: https://issues.apache.org/jira/browse/OPENJPA-759
Project: OpenJPA
Issue Type: Improvement
Components: sql
Affects Versions: 2.0.0
Environment: DB2 UDB
Reporter: Milosz Tylenda
Priority: Minor
Attachments: OPENJPA-759.patch
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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