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https://issues.apache.org/jira/browse/HAWQ-492?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15205889#comment-15205889
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Michael Andre Pearce (IG) commented on HAWQ-492:
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Thanks Goden,
Unfortunately this doesn't resolve my issue.
It covers only being able to push down the predicate WHERE logic to the
external system. And I've been able to achieve this with being able to
re-consititue the WHERE statement for the down stream call to the external
table from the predicates passed of GT, LT, GE, LE, etc.
In the example it seems a full row is still returned. And indeed this is
wanting to avoid.
Say in HAWQ you submit SELECT a, b FROM external_mysql_table WHERE z < 3 AND x
> 6
I can construct now SELECT * FROM mysql_table WHERE z < 3 AND x > 6 and push
this down into the downstream db, with pushdown on and filters applied.
Alas as you notice i am still having to return the full row, even so the HAWQ
select only needs a and b, as such should look to only pass these columns back
to HAWQ for effieciencies. I want to be able to construct the SELECT a, b part
also.
>From the documents and sample do not see how to achieve knowing i need only
>SELECT columns a and b, only seems to cover how to create the predicate WHERE
>clause.
Thanks.
> PXF Query Push-Down Feature enhancement to enable further push down to
> Datasource
> ---------------------------------------------------------------------------------
>
> Key: HAWQ-492
> URL: https://issues.apache.org/jira/browse/HAWQ-492
> Project: Apache HAWQ
> Issue Type: Improvement
> Components: PXF
> Reporter: Michael Andre Pearce (IG)
> Assignee: Lei Chang
>
> PXF currently supports push down of filters to the PXF plugin. So that PXF
> itself only needs to return the fields and rows that match the filters.
> Unfortunately it seems (and it could be simply miss-understanding) is done in
> a way where by the PXF plugin must retrieve the complete row/entry from the
> source then the filters are applied. With Query Filter Push-Down feature.
> With a JDBC PXF (https://github.com/kojec/pxf-field/tree/master/jdbc-pxf-ext)
> this seems to be the case where by the SELECT * FROM table is done.
> It would be much more efficient if the filter and fields are push down in a
> way so that the plugin can if in its use case can actually delegate that
> further down into datasource it is using, and retrieving and applying the
> filters needed to reduce the data it needs to be pulled back.
> As such the PXF accessor would be able to construct SELECT fieldA fieldB FROM
> table WHERE fieldA > 6
> It maybe that this is indeed possible, but looking at
> https://github.com/kojec/pxf-field/tree/master/jdbc-pxf-ext and the API's and
> documents currently available we cannot see a way to improve this jdbc pxf
> plugin to achieve this.
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