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From "Aaron Kimball (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-840) DBInputFormat leaves open transaction
Date Mon, 10 Aug 2009 08:06:14 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-840?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12741220#action_12741220
] 

Aaron Kimball commented on MAPREDUCE-840:
-----------------------------------------

Enis,

Closing the resultset and the statement do not commit the transaction. The postgresql server
will continue to hold reader locks on all affected tables until that connection is closed,
times out, etc. (Postgresql will begin a transaction for any SELECT statement; other drivers
may or may not wrap reads in a transaction -- JDBC doesn't mandate what happens here. Thus
far this is an issue for me only with Postgresql; mysql and oracle don't seem to produce this
same behavior.)

This is about cleaning up server-side resources, at least insofar as Postgresql implements
the spec. I think that it's good hygiene to assume that any driver may open a transaction
that should be committed when we're done reading.

> DBInputFormat leaves open transaction
> -------------------------------------
>
>                 Key: MAPREDUCE-840
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-840
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>            Reporter: Aaron Kimball
>            Assignee: Aaron Kimball
>            Priority: Minor
>         Attachments: MAPREDUCE-840.patch
>
>
> DBInputFormat.getSplits() does not connection.commit() after the COUNT query. This can
leave an open transaction against the database which interferes with other connections to
the same table.

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