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From "Kristian Waagan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3732) SQL Length function materializes lob into memory
Date Fri, 20 Jun 2008 08:59:45 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3732?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12606676#action_12606676
] 

Kristian Waagan commented on DERBY-3732:
----------------------------------------

Of course this bug should be fixed, but as Mike suggested in another Jira issue, I think it's
time to look a bit closer on the code and see if we need a more thorough rewrite. The SQLXX
classes themselves comes to mind, but we should also verify that there isn't a mismatch between
what the SQLXX classes expect/assumes and what the store is capable of. Last, we also have
a number of LOB specific classes on top.

For instance, would it be wiser to more clearly separate code for LOBs from the smaller data
types in the data type hierarchy?
I don't know the history of the code, but it might be problematic if the existing code was
written with small data types in mind.

There seems to be at least two separate issues:
 a) Materialization
 b) Stream handling

As a sub point under b, I'm also wondering why we don't encode stream length in the store
stream and how hard it would be to fix this. One problem is that the space reserved for this
purpose seems too limited.

> SQL Length function materializes lob into memory
> ------------------------------------------------
>
>                 Key: DERBY-3732
>                 URL: https://issues.apache.org/jira/browse/DERBY-3732
>             Project: Derby
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 10.3.3.0, 10.4.1.3, 10.5.0.0
>            Reporter: Kathey Marsden
>            Priority: Minor
>         Attachments: LengthLargeLob.zip
>
>
> Currently the SQL length function materializes the entire lob into memory. In SQLBinary.getLength()
we have 
> public final int	getLength() throws StandardException
> 	{
> 		if (stream != null) {
> 			if (streamValueLength != -1)
> 				return streamValueLength;
> 		}
> 		return (getBytes() == null) ? 0 : getBytes().length;
> 	}
> Which actually is doubly bad because we call getBytes twice and materialize it twice.
> It would be good to read the length from the stream if available and otherwise stream
the value to get the length, rather than materializing it into memory.
> To reproduce, run the attached repro.
> java -Xmx16M  LengthLargeLob
> It gives an out of memory exception
> Caused by: java.lang.OutOfMemoryError: Java heap space
>         at org.apache.derby.iapi.types.SQLBinary.readFromStream(SQLBinary.java:415)
>         at org.apache.derby.iapi.types.SQLBinary.readExternal(SQLBinary.java:318)
>         at org.apache.derby.iapi.types.SQLBinary.getValue(SQLBinary.java:220)
>         at org.apache.derby.iapi.types.SQLBinary.getBytes(SQLBinary.java:210)
>         at org.apache.derby.iapi.types.SQLBinary.getLength(SQLBinary.java:250)
>         at org.apache.derby.impl.sql.execute.BaseActivation.getDB2Length(BaseActivation.java:1684)
>         at org.apache.derby.exe.acf81e0010x011axa317x5db8x0000003d9dc81.e1(Unknown Source)
>         at org.apache.derby.impl.services.reflect.DirectCall.invoke(ReflectGeneratedClass.java:141)
>         at org.apache.derby.impl.sql.execute.ProjectRestrictResultSet.doProjection(ProjectRestrictResultSet.java:497)
>         at org.apache.derby.impl.sql.execute.ProjectRestrictResultSet.getNextRowCore(ProjectRestrictResultSet.java:291)
>         at org.apache.derby.impl.sql.execute.BasicNoPutResultSetImpl.getNextRow(BasicNoPutResultSetImpl.java:460)
>         at org.apache.derby.impl.jdbc.EmbedResultSet.movePosition(EmbedResultSet.java:423)
>         ... 2 more
> [
>  

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