[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: derby1482_patch2_stat.txt
                derby1482_patch2_diff.txt

Attaching patch, derby1482_patch2_diff.txt, which is very similar to the 1st patch except that rather than adding a new column to SYSTRIGGERS to save the trigger action columns information, it expands the functionality of ReferenceColumn implementation to include trigger action columns. This will hopefully make it easier upgrade path. The only surprise I ran into so far is that ReferenceColumn gets used by 2 more system tables namely SYSCHECKS
and SYSCONSTRAINTS which means we need to make sure that upgrade code works for those system tables too. Next, I will start looking at the upgrade code. I think, the upgrade work that needs to go in for DERBY-1482 will be similar to what was done for DERBY-606
(committed with revision number 484797). I will study that patch more to see how it's logic can be used for the changes to SYSTRIGGERS.REFERENCEDCOLUMNS needed for this jira.

> 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, 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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