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From "Satheesh Bandaram (JIRA)" <derby-...@db.apache.org>
Subject [jira] Commented: (DERBY-442) Update triggers on tables with blob columns stream blobs into memory even when the blobs are not referenced/accessed.
Date Thu, 13 Apr 2006 18:04:01 GMT
    [ http://issues.apache.org/jira/browse/DERBY-442?page=comments#action_12374387 ] 

Satheesh Bandaram commented on DERBY-442:
-----------------------------------------

Thanks Manish for some research on this. Yes, this has been a known troublesome area for sometime.
The problem gets worse if there are any BLOB/CLOB columns in the table. Even if these columns
are not referenced in the trigger, current logic materializes the whole BLOB/CLOB into memory.

According to Derby documentation, REFERENCEDCOLUMNS is ' true descriptor of the columns referenced
by UPDATE triggers'. REFERENCEDCOLUMNS is used to keep a list of columns that trigger the
update. What you are proposing need another column that keeps track of all columns used in
the trigger, not just triggering columns. That would be good to avoid materializing unreferenced
columns, but we may also need to avoid blob/clob materialization to make them stream on-demand.
There is another issue with supporting BLOB/CLOBs in triggers.

> Update triggers on tables with blob columns stream blobs into memory even when the blobs
are not referenced/accessed.
> ---------------------------------------------------------------------------------------------------------------------
>
>          Key: DERBY-442
>          URL: http://issues.apache.org/jira/browse/DERBY-442
>      Project: Derby
>         Type: Sub-task

>   Components: SQL
>     Versions: 10.2.0.0
>     Reporter: A B
>     Priority: Minor
>      Fix For: 10.2.0.0
>  Attachments: d442.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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