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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 01:18:50 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2380?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12476806
] 

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

First, some cleanup is needed for stored prepared statements (which are used for JDBC meta-data
queries and triggers).
ExecSPSNode has this code (generate)

		/*
		** The following does a prepare on the underlying
		** statement if necessary.  The returned statement
		** is valid and its class is loaded up.
		*/
		ps = spsd.getPreparedStatement();

However the returned statement is "partially valid", it's isValid field is set to false because
SPSDescriptor performed a makeInvalid() on it when it compiles the stored plan the first time.
SPSDescriptor then holds onto this "partially valid" to provide the compiled information for
the real running of the statement through the internal  EXECUTE STATEMENT command. Applying
the obvious fix for this bug, nulling out the compiled information, then leaves the prepared
statement used by SPS in a fully invalid state, causing NullPointerExceptions as all the compiled
information has been removed.

Having a "partially valid" state is not a good design, it complicates the already hard to
understand SPS code. Will be looking at ways to avoid this so that a makeInvalid() on a language
prepared statement (plan) can always make the object invalid.

> 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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