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https://issues.apache.org/jira/browse/DERBY-1482?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Mamta A. Satoor updated DERBY-1482:
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Attachment: derby1482DeepCopyAfterTriggerOnLobColumn.java
I have been spending some time on this jira writing test cases and researching
the code a bit. Also spoke to Mike about the jira in general. Here are the
findings so far.
Following is the Derby behavior that we expect in regards to the lob columns
and update triggers
1)In case of an AFTER UPDATE TRIGGER on the LOB column, we expect Derby to do a
deep copy of the before and after image of LOB columns in the triggering table.
The reason for deep copy is the contents of LOB will be different in store
before the update and after the update. I have a java program conforming this
behavior. That java program is attached and the program's name is
derby1482DeepCopyAfterTriggerOnLobColumn.java
2)If the AFTER UPDATE TRIGGER is defined on non-LOB column, Derby still does a
deep copy of before and after image of LOB columns in the triggering table. The
possible improvements that can be made to Derby is to detect somehow if there
is any need to do deep copy at all if the LOB columns are not getting accessed
in the TRIGGER. If they are never accessed, there is no need to any
before/after update values of the LOB columns thus saving memory requirement
and unnecssary time spent in doing copy. I am working on writing a test case
conforming that deep copy is being done even though there is really no need to
since LOB columns are not getting accessed.
3)In case of BEFORE UPDATE TRIGGER, we should not be doing any deep copy
because the before and after images of LOB should be same (which will be the
LOB value before update was done). I am working on writing test case for this
scenario too.
> Update triggers on tables with blob columns stream blobs into memory even
> when the blobs are not referenced/accessed.
> ---------------------------------------------------------------------------------------------------------------------
>
> Key: DERBY-1482
> URL: https://issues.apache.org/jira/browse/DERBY-1482
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.2.1.6
> Reporter: Daniel John Debrunner
> Priority: Minor
> Attachments: derby1482DeepCopyAfterTriggerOnLobColumn.java,
> derby1482Repro.java
>
>
> Suppose I have 1) a table "t1" with blob data in it, and 2) an UPDATE trigger
> "tr1" defined on that table, where the triggered-SQL-action for "tr1" does
> NOT reference any of the blob columns in the table. [ Note that this is
> different from DERBY-438 because DERBY-438 deals with triggers that _do_
> reference the blob column(s), whereas this issue deals with triggers that do
> _not_ reference the blob columns--but I think they're related, so I'm
> creating this as subtask to 438 ]. In such a case, if the trigger is fired,
> the blob data will be streamed into memory and thus consume JVM heap, even
> though it (the blob data) is never actually referenced/accessed by the
> trigger statement.
> For example, suppose we have the following DDL:
> create table t1 (id int, status smallint, bl blob(2G));
> create table t2 (id int, updated int default 0);
> create trigger tr1 after update of status on t1 referencing new as n_row
> for each row mode db2sql update t2 set updated = updated + 1 where t2.id =
> n_row.id;
> Then if t1 and t2 both have data and we make a call to:
> update t1 set status = 3;
> the trigger tr1 will fire, which will cause the blob column in t1 to be
> streamed into memory for each row affected by the trigger. The result is
> that, if the blob data is large, we end up using a lot of JVM memory when we
> really shouldn't have to (at least, in _theory_ we shouldn't have to...).
> Ideally, Derby could figure out whether or not the blob column is referenced,
> and avoid streaming the lob into memory whenever possible (hence this is
> probably more of an "enhancement" request than a bug)...
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