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https://issues.apache.org/jira/browse/DERBY-5696?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13261296#comment-13261296
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Kristian Waagan commented on DERBY-5696:
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Hi Kim,
The base rule is that you can only access a LOB column once. To access a column
you invoke a getter on it.
The exceptions to that rule are getBytes and getString, which allow you to
invoke those two getters as many times as you like, given none of the other
getters has been invoked already, and additionally one other getter as the last
invocation.
The reason for the exception to the rule is to avoid breaking existing
applications that work correctly. getBytes and getString ([1]) are different
because they always materialize the value. With materialized values you don't
have the problems with stream positioning that you have [in Derby] when serving
content off a store stream.
The fact that you can invoke getBytes/getString multiple times is not really an
important thing to note in the manual. What users need to know is that you can
only invoke a getter once on a LOB column. If we document the special behavior
of getBytes/getString, it is to allow user to understand the behavior they are
seeing in their application. Some users may use/abuse that for convenience -
that's fine as long as they know their large objects are small object :)
Don't hesitate to ask further questions if my explanation is unclear.
[1] ResultSet.get[Bytes|String] and Blob.getBytes/Clob.getSubString are
different as the former alway fetch the whole value, whereas the latter allows
for fetching only parts of the value.
> Documentation on LOBs needs some fixes
> --------------------------------------
>
> Key: DERBY-5696
> URL: https://issues.apache.org/jira/browse/DERBY-5696
> Project: Derby
> Issue Type: Bug
> Components: Documentation
> Affects Versions: 10.8.2.2
> Reporter: Kim Haase
> Assignee: Kim Haase
>
> DERBY-5489 points out some issues with multiple getXXX calls on LOBs that are
> not fully documented. The information should probably be added to "Notes on
> mapping of java.sql.Blob and java.sql.Clob interfaces". In addition, the
> topic "Mapping of java.sql.Blob and java.sql.Clob interfaces" has a typo and
> could probably use some additional information as well.
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