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From "Kristian Waagan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-5698) Document performance issue with 2-arg versions of setXXXStream methods for LOBs
Date Fri, 11 May 2012 06:18:44 GMT

    [ https://issues.apache.org/jira/browse/DERBY-5698?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13273070#comment-13273070
] 

Kristian Waagan commented on DERBY-5698:
----------------------------------------

Derby uses a header to store length information for LOBs. If this header can't be filled in
when the LOB is inserted, which will be the case if the LOB is inserted without specifying
the length up front and the LOB is larger than a single page (typically 32 KB, but may be
smaller too), Derby has no mechanism to update the header without rewriting the whole LOB
at a later time.

The insert performance will be unaffected ([1]). You'll see impaired performance if you start
asking for the length of these LOBs, as Derby has to fetch and decode the whole value to find
the length.
If you're just reading data, say, with a stream, performance is unaffected.

Assuming you inserted 101 LOBs without specifying their lengths, 100 of them being 2 GB big
and one only 10 bytes, you'd be in a world of hurt with the following query:
    select min(length(myLOBs)) from mytable

I think this can be fixed, but it will require some non-trivial work in the store.


[1] For recent versions, there were some edge-case issues with older versions using the client
driver.
                
> Document performance issue with 2-arg versions of setXXXStream methods for LOBs
> -------------------------------------------------------------------------------
>
>                 Key: DERBY-5698
>                 URL: https://issues.apache.org/jira/browse/DERBY-5698
>             Project: Derby
>          Issue Type: Improvement
>          Components: Documentation
>    Affects Versions: 10.8.2.2
>            Reporter: Kim Haase
>            Assignee: Kim Haase
>         Attachments: DERBY-5698.diff, DERBY-5698.stat, DERBY-5698.zip
>
>
> The PreparedStatement.setAsciiStream and other methods have a 3-arg form that includes
the length of the stream and a 2-arg form that does not. If the 2-arg form is used, Derby
has to calculate the length every time the method is called. With LOBs, especially large ones,
this can cause a major performance impact, especially if the method is called repeatedly.
This should be documented where appropriate.
> Kristian, please feel free to correct or amplify anything I've said here.

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