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From mike matrigali <mikema...@gmail.com>
Subject Re: Store api question: how to ask for RowLocations
Date Tue, 01 Oct 2013 22:40:51 GMT
In general fetchLocation is available for "normal" scans, but obviously
not available for when store returns bulk set of rows back in the form
of a hash table.  also GroupFetchScanController have special interfaces
to return the RowLocation.

Is your problem specific to hash table returns, or are there other cases
where fetchLocation from a ScanController does not work?

On 10/1/2013 10:29 AM, Rick Hillegas wrote:
> I need some advice about how to design an api for requesting that the
> Store include RowLocations in the rows that it scans and hands back to
> the language layer.
>
> The immediate problem that I'm working on involves implementing the
> MERGE statement (DERBY-3155). Part of the implementation involves
> cooking up a left join between two tables. I need to get back
> RowLocations for the right table of that join. In a particular problem
> case which I'm examining, the optimizer picks a HashJoin strategy for
> the left join. That turns into a HashLeftOuterJoinResultSet at execution
> time. And that, in turn, involves having the Store create and fill a
> BackingStoreHashTableFromScan.
>
> The BackingStoreHashTableFromScan is created with a scanColumnList (a
> FormatableBitSet) which specifies some actual columns in the row as well
> as a trailing column position which is meant to represent the
> RowLocation. That trailing column position is represented as 1 plus the
> actual row length. BackingStoreHashTableFromScan doesn't know what to
> make of that column position and silently ignores it. So clearly either
> that's the wrong api for asking for RowLocations or
> BackingStoreHashTableFromScan needs to be taught some new tricks.
>
> So the question is this: what's the right way to ask
> BackingStoreHashTableFromScan to build a hash table whose rows contain
> some set of real column positions plus a trailing RowLocation column? I
> may stumble into other situations where I need to ask a scan to put
> RowLocations into the rows it returns. So it would be good to have a
> general pattern here for requesting this special column.
>
> Thanks,
> -Rick
>
>


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