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