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From "Kristian Waagan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-5696) Documentation on LOBs needs some fixes
Date Wed, 25 Apr 2012 04:50:20 GMT

    [ https://issues.apache.org/jira/browse/DERBY-5696?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13261296#comment-13261296
] 

Kristian Waagan commented on DERBY-5696:
----------------------------------------

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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