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Date: Fri, 28 May 2010 14:32:36 -0400 (EDT)
From: "Mike Matrigali (JIRA)" <jira@apache.org>
To: derby-dev@db.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.
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     [ https://issues.apache.org/jira/browse/DERBY-1482?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

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


I just read back a bit and see the issue about SERIALIZATION across client/server.  Do you know what JIRA caused this behavior change in 10.6?  I am not sure if it is even possible to do a proper 
soft/hard upgrade on anything in a system catalog that could properly take account of client/server
mismatches.

I would lean toward documenting that we don't support this access, it seems like we should only 
support using the standard jdbc data dictionary interfaces. 

I was not aware of this new client/server behavior.  One of the reasons for the current design in this fix, was to avoid the complication of creating new format id's and subclasses.  And as you point out even
if we do this we can't guarantee anything other than a format id not found error on the backward client since in the worst case it can't have any new code to handle new stuff in hard upgraded servers.  

> 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: derby1482_patch1_diff.txt, derby1482_patch1_stat.txt, derby1482_patch2_diff.txt, derby1482_patch2_stat.txt, derby1482_patch3_diff.txt, derby1482_patch3_stat.txt, derby1482DeepCopyAfterTriggerOnLobColumn.java, derby1482Repro.java, derby1482ReproVersion2.java, junitUpgradeTestFailureWithPatch1.out, 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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