[jira] Updated: (DERBY-1482) Update triggers on tables with blob columns stream blobs into memory even when the blobs are not referenced/accessed.

["Mamta A. Satoor (JIRA)" <ji...@apache.org>]

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

Mamta A. Satoor updated DERBY-1482:
-----------------------------------

    Attachment: derby1482Repro.java

Including the steps here to reproduce the problem of out of memory.
1)First create the database with 3 populated tables using the attached java program derby1482Repro.java as follows
java org.apache.derbyTesting.functionTests.tests.lang.derby1482Repro
Some information about the 3 tables
   a)Table t1 has a BLOB column along with other columns.
   b)Table t3 is similar to t1 but without a BLOB column
   c)Table t1 has an update trigger defined on it which updates table t2 in the trigger action.
   d)Table t3 also has an update trigger defined on it which also updates table t2 in the trigger action.
2)To reproduce the problem of OutOfMemory, connect to the database using ij with very limited heap
java -Xmx4m -Dij.exceptionTrace=true org.apache.derby.tools.ij
connect 'jdbc:derby:testDB1482;create=true';
--following update of table t3 works with no problem. This is the table with no BLOB column
update t3 set status = 1 where id = 1;
--But the following update will fail with OOM. This update is on the table with BLOB column
update t1 set status = 1 where id = 1;

The attached derby1482Repro.java is subset of the repro provided by Army for DERBY-442.


> 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: 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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