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https://issues.apache.org/jira/browse/DERBY-1482?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12836742#action_12836742
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Mamta A. Satoor commented on DERBY-1482:
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I wrote following ij script to understand what kind of information is kept in
SYSTRIGGERS when REFERENCING clause is specified or missing in the CREATE
TRIGGER sql. There are 2 booolean columns, namely, REFERENCINGOLD and
REFERENCINGNEW in SYSTRIGGERS table which are set to false if REFERENCING is
missing. If either one of the REFERENCING clauses are specified (for OLD and
NEW), the corresponding columns' values will be set to TRUE, We can hopefully
use this information to disable streaming of LOB columns. I will look at the
code to see how this information can be used
java -Dij.exceptionTrace=true org.apache.derby.tools.ij
connect 'jdbc:derby:testDB1482;create=true';
create table table1 (c11 int, c12 int default 0);
create index i1 on table1(c11);
create table table2 (c21 int, c22 int default 0);
create index i2 on table2(c21);
--For the trigger below, REFERENCINGOLD and REFERENCINGNEW will be false.
-- REFERENCINGOLD and REFERENCINGNEW are false because no REFERENCING clause
was used in the CREATE TRIGGER.
create trigger trigger1 AFTER INSERT on table1 insert into table2(c21, c22)
values (100, -1);
--For the trigger below, REFERENCINGNEW will be true because CREATE TRIGGER
used REFERENCING NEW.
-- REFERENCINGOLD will be false because no REFERENCING OLD was specified in the
CREATE TRIGGER sql.
create trigger trigger2 AFTER UPDATE of c12 on table1 REFERENCING NEW as new
FOR EACH ROW
update table2 set c22=-1 where c21=new.c11;
select CAST(TRIGGERNAME as char(14)), REFERENCEDCOLUMNS, TRIGGERDEFINITION,
REFERENCINGOLD, REFERENCINGNEW from sys.systriggers;
> 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: 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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