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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-1482) Update triggers on tables with blob columns stream blobs into memory even when the blobs are not referenced/accessed.
Date Sat, 27 Feb 2010 02:12:05 GMT

     [ https://issues.apache.org/jira/browse/DERBY-1482?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Mike Matrigali updated DERBY-1482:
----------------------------------


I think reading it again from the store is a much better approach.  Best if the mechanism
can be somehow hidden withing the stream that the is passed from the
store up to the user.

This may or may not solve the various trigger issues, as the trigger needs a specific
version of the row which might change between when the stream is returned and when it is asked
for.  For instance an update trigger must provide the before and after version of the value,
but the trigger action itself may change the row.  This may not be an issue if the getting
it "from the store" in this case is actually getting it from a deffered update temporary table,
then there would be no worry about the trigger action changing it.

I think this is easier than the log approach, but not a simple change.  I was thinking of
taking a look at this but would rather let you get all your stream changes in to see what
problems remain.  

> 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
>            Assignee: Mamta A. Satoor
>            Priority: Minor
>         Attachments: derby1482DeepCopyAfterTriggerOnLobColumn.java, derby1482Repro.java,
derby1482ReproVersion2.java, TriggerTests_ver1_diff.txt, TriggerTests_ver1_stat.txt
>
>
> 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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