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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2822) Add caching of store stream length in StoreStreamClob, if appropriate
Date Tue, 28 Oct 2008 13:18:44 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2822?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12643196#action_12643196
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Knut Anders Hatlen commented on DERBY-2822:
-------------------------------------------

I think this patch is OK. It wasn't immediately clear to me that it was correct to set the
variable once and never update it or invalidate it, but it seems like the StoreStreamClob
object isn't changed when the underlying Clob changes, even without your patch, so it looks
safe to me.

> Add caching of store stream length in StoreStreamClob, if appropriate
> ---------------------------------------------------------------------
>
>                 Key: DERBY-2822
>                 URL: https://issues.apache.org/jira/browse/DERBY-2822
>             Project: Derby
>          Issue Type: Improvement
>          Components: JDBC
>    Affects Versions: 10.3.1.4
>            Reporter: Kristian Waagan
>         Attachments: derby-2822-1a-cacheCharLength.diff
>
>
> Currently StoreStreamClob reads the whole Clob stream, including decoding it, to find
the length of it. It also does this the second time the length is asked for.
> StoreStreamClob is the internal Clob representation for Clobs that are read-only. As
soon as the user updates the Clob, it is transferred to a modifiable Clob representation.
> It should be determined if it is safe to cache the length (both in bytes and in characters)
of the store stream to improve the performance and reduce the load on Derby for certain Clob
operations.
> To do this, the locking mechanism used for Clobs must be analyzed.
> If you have obtained a Clob object, is there a lock in place that stops others from changing
the content?
> For all isolation levels?
> What about scrollable result sets?
> Can the streamed content from store be changed under us, and thus invalidate a cached
length?

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