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From "Daniel John Debrunner (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2380) A statement plan holds onto resources such as its generated class even after it has been invalidated.
Date Thu, 01 Mar 2007 13:37:52 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2380?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12476933
] 

Daniel John Debrunner commented on DERBY-2380:
----------------------------------------------

A GenericPreparedStatement instance can used in various situations and the resuling handling
of its state in these situations is awkward.
The various situations are in the statement cache and unused, in the cache and in use, referenced
in a JDBC prepared statement with an activation, attached to an SPSDescriptor without an activation,
in-use in a trigger.
The state handling involves a use-count (for existing activations), the check to see if the
plan is being cache and the "partially valid" state for the SPS mentioned in the previous
comment.

Ideally just having a reference to a GenericPreparedStatement would keep it valid.

Looking at ways to clean this up it seems that the root cause is the DependencyManager. The
DependencyManager is implementing the Observer pattern but in a non-java way.
http://www.research.umbc.edu/~tarr/dp/lectures/Observer.pdf

Since the GenericPreparedStatement instance is held onto in a central DependencyManager object,
then some state is needed as to when it should be removed from that list,handling dependencies
the "java way" would instead mean the list of dependents is held in the object itself, not
a central list.
Now the Java Observable class has some issues, but I wonder if the pattern should be applied
here. Persistent dependencies cause some issues but I just wanted to throw my thoughts out
there.


> A statement plan holds onto resources such as its generated class even after it has been
invalidated.
> -----------------------------------------------------------------------------------------------------
>
>                 Key: DERBY-2380
>                 URL: https://issues.apache.org/jira/browse/DERBY-2380
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 10.0.2.0, 10.0.2.1, 10.1.1.0, 10.1.2.1, 10.1.3.1, 10.2.1.6, 10.2.2.0,
10.3.0.0
>            Reporter: Daniel John Debrunner
>         Assigned To: Daniel John Debrunner
>
> An internal plan (instance of GenericPreparedStatement) can be invalidated by other SQL
operations such as DROP TABLE or DROP INDEX.
> When this happens the references to objects that are no longer useful such as the generated
class and saved objects are held onto and thus use memory.
> If the statement is re-compiled then these objects will be handled by garbage collection.
> If the statement is not recompiled though, then these objects will remain until the plan
(GenericPreparedStatement) is garbage collected.
> The plan being garbage collected can be held up for two reasons:
>    1) The plan is in the statement cache. Note that only in some cases does it make sense
to remove an invalid plan from the statement cache, e.g. a DROP TABLE should remove any plan
that uses that table, but a DROP TRIGGER should not remove an INSERT from the cache, as it
is likely the plan will be re-used and re-compiled. This  is a separate issue given that the
memory usage can occur even if the plan is not in the cache.
>    2) The application holds onto a JDBC PreparedStatement that uses the plan.
> Given an application should not be able to affect memory usage like this then the GenericPreparedStatement.makeInvalid()
call should null out fields that hold references to objects that have become invalid.

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