Update triggers on tables with blob columns stream blobs into memory even when
the blobs are not referenced/accessed.
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Key: DERBY-1482
URL: http://issues.apache.org/jira/browse/DERBY-1482
Project: Derby
Type: Bug
Components: SQL
Versions: 10.2.0.0
Reporter: Daniel John Debrunner
Priority: Minor
Fix For: 10.2.0.0
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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