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From "Dyre Tjeldvoll (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-827) Performance can be improved by re-using language ResultSets across Activation executions.
Date Fri, 02 Mar 2007 10:17:50 GMT

    [ https://issues.apache.org/jira/browse/DERBY-827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12477244
] 

Dyre Tjeldvoll commented on DERBY-827:
--------------------------------------

Thanks for looking at the test Knut-Anders. I'll try to make a
followup-patch which addresses your comments.

I did some more poking around in a debugger and I now think that
materialization of subqueries whose values are used as qualifiers
in table scans, is unnecessary. The qualifier actually caches the
value itself after the first invocation of the byte code, so
optimizing the byte code to return a cached value also, seems
redundant. This may be different in cases where the subquery isn't
used as a qualifier.

I think the byte code generation for sub queries must be changed so
that it matches the changes in derby827_update920.txt, and keeps a
reference to internal resultsets created by the subquery during the
initial execution. Subsequent executions would only check for a valid
result set tree and then return. That way the resultsets from the
subquery can be reused also.

I'm not quite sure what should go in the Activation.execute() stage,
and what should go into the ResultSet.open() stage. It seems like the
execute() stage is responsible for creating the resultset tree, and
that ResultSet.open() should propagate to open() (openCore()) calls
down the tree? That would give the execute() stage very little to do
when the result sets are already there? Anyway, with this strategy
TableScanResultSet.close() would invalidate the qaulifier cache, and
the next use of the qualifier would trigger a sequence of open calls
to the subquery result set tree.

Materialization of subqueries becomes a problem here, beacuse with it
one needs some way of invalidating the materialization (as well as the
qualifier cache) when a single execution is complete. Assuming that's
doable somehow, which stage should then be responsible for placing a
new qualifier inside the TableScanResultSet? Seems kind of awkward to
let rs.open() manipulate its own state that way. On the other hand,
letting the execution stage re-open the subquery result set tree to
get the new materialization and stuff into the existing
TableScanResultSet doesn't seem very clean either...

Feel free to provide opinions/comments/suggestions/advice...

> Performance can be improved by re-using language ResultSets across Activation executions.
> -----------------------------------------------------------------------------------------
>
>                 Key: DERBY-827
>                 URL: https://issues.apache.org/jira/browse/DERBY-827
>             Project: Derby
>          Issue Type: Improvement
>          Components: Performance
>            Reporter: Daniel John Debrunner
>         Attachments: derby827_draft_reuse_result_sets.txt, derby827_update920.txt, rsfromps.v1.diff,
rsfromps.v1.stat, rsfromps_prelim.diff, rsfromps_prelim2.diff
>
>
> >Shouldn't DistinctScalarAggregateRS implement a close or a finish method
> >>(not sure what the difference is) and close the scan controller there.
> The close() and finish() methods are actually explained in their javadoc
> in the language org.apache.derby.iapi.sql.ResultSet class.
> [note this is not a JDBC java.sql.ResultSet object]
> close() -  Tells the system that there will be no more calls to
> getNextRow() (until the next open() call)
> finish() - Tells the system that there will be no more access to any
> database information via this result set
> So close means the ResultSet may be opened again for more access, while
> finish means it will not be used again.
> However, their use in the code always doesn't match that, and that does
> cause confusion, at least to me.
> Language ResultSets (not JDBC ones) can be and are opened multiple
> times, for example when scanning a table multiple times within a join.
> An Activation, which represents the internal state of
> java.sql.PreparedStatement object & has the lifetime of the
> java.sql.PreparedStatement, contains a top-level language ResultSet.
> This top-level language ResultSet provides the execution of the SQL
> statement, DML, DDL or a query. The top-level ResultSet may contain
> other ResultSets and could be seen as a tree structure. For the simple
> case of a primary key lookup query like:
>    select name from customer where id = ?
> The activation would contain this:
> top result set
> ProjectRestrictRS << IndexRowToBaseRowRS << TableScanRS
> Now for some reason, even though the api of ResultSet say they can be
> re-used, and in some cases they are, this result set tree is thrown away
> after each execution. That is, the top result set has its finish()
> method called and then the activation removes its reference to it. Then
> on the next execution a new (identical) tree is set up.
> There is potential for a huge performance gain if this top level result
> set and its tree are re-used and have the same lifetime as the
> Activation. The saving comes in two forms, not having to create many
> objects on each execution, and not creating short-lived objects for the
> garbage collector to handle.
> I made a simple fix, it's a couple of lines of code, just calling close
> & finish at the correct times, and for the above simple primary key
> lookup query, the performance went from 17,300 to 24,000 selects per
> second (cached data, single user). I'll post a patch shortly as an
> indication of the direction, once I can separate it from other changes
> in my client.
> However, I'm running the Derby tests and there are some (maybe 25-30)
> failures, I think because not all the language ResultSet implementations
> are correctly written to be re-opened. Interestingly, the first failure
> I saw was in an aggregrate test, which goes back to the issue Manish saw.
> Even if derbyall passed I would be nervous about submitting this patch
> for real, because I don't think there's a lot of testing using repeat
> executions of PreparedStatements in the tests. The ij tests mainly use
> Statement, this is a single use of an activation so this change would
> not affect them. Thus such a patch could regress Derby by making it more
> likely existing bugs would be exposed.
> Given the performance gains, I think we need to start re-using
> ResultSets from Activation, and devise a way to ensure the testing
> covers the re-use. The main issue is there is a large number of
> ResultSet implementations to cover.

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