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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-3907) Save useful length information for Clobs in store
Date Wed, 05 Nov 2008 18:09:44 GMT

     [ https://issues.apache.org/jira/browse/DERBY-3907?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Mike Matrigali updated DERBY-3907:
----------------------------------


The updating discussion has been a bit confusing.   I am concentrating on the store interface
to update an object on disk.  I realize there are a number of in memory clob update paths,
but that is a different issue.  Currently there is no way to update "part" of the object in
the store
interface, only the whole object.

So without store changes your normal use case for the lengthless updates would require the
entire clob to be rewritten to disk.  

I think easiest would be to support a new store interface that allowed the update of first
initial n bytes by an exact same number of bytes.  Currently for long clobs the usual case
would be that we used all the bytes on the 1st page for the fist piece of the linked list,
so expanding by any number of bytes would be a worst case.  We would log at least the entire
"old" portion of the row.  If the lengths don't exactly match there is more logging complication.
 So my guess at ease of implementation from easiest to hardest supporting some subset of bytes
update:
o exact leading number of bytes
o shrinking leading number of bytes
o expanding leading number of bytes
o arbitrary positioned number of bytes

This probably would mean a new log record, and thus need to be careful again about hard/softupgrade.
 We have done log record updates (or new ones) in derby so an example
exists.

> Save useful length information for Clobs in store
> -------------------------------------------------
>
>                 Key: DERBY-3907
>                 URL: https://issues.apache.org/jira/browse/DERBY-3907
>             Project: Derby
>          Issue Type: Improvement
>          Components: JDBC, Store
>    Affects Versions: 10.5.0.0
>            Reporter: Kristian Waagan
>            Assignee: Kristian Waagan
>
> The store should save useful length information for Clobs. This allows the length to
be found without decoding the whole data stream.
> The following thread raised the issue on what information to store, and also contains
some background information: http://www.nabble.com/Storing-length-information-for-CLOB-on-disk-tp19197535p19197535.html
> The information to store, and the exact format of it, is still to be discussed/determined.
> Currently two bytes are set aside for length information, which is inadequate.

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