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From "Mamta A. Satoor (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-1482) Update triggers on tables with blob columns stream blobs into memory even when the blobs are not referenced/accessed.
Date Tue, 23 Feb 2010 06:45:28 GMT

    [ https://issues.apache.org/jira/browse/DERBY-1482?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12837104#action_12837104
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Mamta A. Satoor commented on DERBY-1482:
----------------------------------------

One of the questions that has come up in this jira is are we reading all the columns when
there is a trigger defined on a table with LOB column(s). I found following relevant code
comment in impl.sql.compile.UpdateNode:getUpdateReadMap which confirms that we indeed are
reading all the columns any time there is a trigger involved on a table involved in definitely
an UPDATE DML.
		/*
	 	** If we have any triggers, then get all the columns
		** because we don't know what the user will ultimately
		** reference.
	 	*/
The job of getUpdateReadMap as the method javadoc says
	  *	Builds a bitmap of all columns which should be read from the
	  *	Store in order to satisfy an UPDATE statement.
But as soon as we find that there are triggers involved on the table, we mark all the columns
as required to be read from
the store. This is the comment from the method's javadoc
	  *	6)	if there are any triggers, marks all columns in the bitmap



> 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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