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https://issues.apache.org/jira/browse/DERBY-827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12484487
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A B commented on DERBY-827:
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> I think it is possible to reset these variables in close() so that the output
> is the same as before,
> but it It seem rather silly to always report the number of opens as 1, when
> the result sets
> actually has been opened (and closed) many times.
I tend to agree with this, though I'm not sure what the best way around it
would be...
I think the intent of "number of opens" is to record how many times a result
set was opened for a _single_ execution (which can be more than 1--namely, if
"reopenCore()" is called). So is there any way to tell the difference between
"reopens" that occur within a single execution vs "reopens" that occur because
of multiple executions of the same statement? For example, is it true that
"openCore()" will only be called once per statement execution while
"reopenCore()" may be called multiple times per statement execution? If so,
could we reset numOpens in the "openCore()" method instead of in the "close()"
method (since the latter may be called several times per execution)? My
apologies if that was already covered in earlier discussions; if so, please
feel free to ignore me :)
> Does the optimizer use these numbers in any way? And if so, will it be
> affected by the change?
Based on my very limited experience with result sets (most of which came from
working on DERBY-47), I don't think the optimizer uses "number of opens" nor
"rows seen" in its estimates. But please keep in mind that I could be wrong
here...
The one thing I noticed was that the value of "rows this scan" is, in certain
situations, written to store and *that* value can affect estimates (because the
optimizer can use that to guess how many rows will be returned from a table).
For more see the "setRowCountIfPossible()" method in
exec/TableScanResultSet.java. I don't know the details on how that works but I
don't think "rows seen" nor "number of opens" directly affects the rowsThisScan
value...
But again, that's just speculation based on limited experience. If anyone out
there knows more hopefully s/he will post here...
> 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: close_nofinish.txt, d827_execute_method_cleanup.txt,
> derby-827.extra.diff, derby827_draft_reuse_result_sets.txt,
> derby827_update920.txt, noclose_finish.txt, noclose_nofinish.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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