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

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 10.2.1.6
• Fix Version/s: 10.9.1.0
• Component/s: SQL
• Labels: LOB derby_backport_reject_10_8
• Urgency: Low
• Issue & fix info: High Value Fix, Release Note Needed
• Assignee: Mamta A. Satoor
• Reporter: Daniel John Debrunner
• Created: 06/Jul/06 22:04
• Updated: 17/Jun/13 09:19
• Resolved: 17/May/11 18:59

Description

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)...

Issue Links

• breaks: DERBY-5121 Data corruption when executing an UPDATE trigger
• incorporates: DERBY-442 Update triggers on tables with blob columns stream blobs into memory even when the blobs are not referenced/accessed.
• is blocked by: DERBY-3940 Dropping a column does not drop triggers which mention that column
• is related to: DERBY-5277 Intermittent OutOfMemoryErrors in BasicSetup.testTriggersWithLOBcolumns()
• is related to: DERBY-5294 Enhance compress table to drop and recreate the triggers. This will enable pre-10.9 triggers (after hard upgrade to 10.9) to read only the required columns from the trigger table.
• is related to: DERBY-4537 Update on tables with blob columns streams blobs into memory even when the blobs are not updated/accessed.
• is related to: DERBY-4538 If the CREATE TRIGGER does not have the REFERENCING clause, then there is no need to keep before and after values for the triggering table