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From "Jinfeng Ni (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (DRILL-5384) Sort cannot directly access map members, causes a data copy
Date Mon, 27 Mar 2017 17:20:41 GMT

    [ https://issues.apache.org/jira/browse/DRILL-5384?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15943669#comment-15943669
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Jinfeng Ni edited comment on DRILL-5384 at 3/27/17 5:19 PM:
------------------------------------------------------------

The reason Drill decides to put a project for "customer.id" before sort operator kicks in
: it greatly simplified the code for sort operator, as it does not have to deal with a complex
schema path, without incurring too much overhead (vector transfer happens at batch level.
it's merely reference transfer; no memory copy involved).

It would be great if you can make sort to handle complex schema path directly. However, I
have doubt about such proposal's performance benefit, until the real performance measurement
prove my suspicion is wrong. 
 

 


was (Author: jni):
The reason Drill decides to put a project for "customer.id" before sort operator kicks in
: it greatly simplified the code for sort operator, as it does not have to deal with a complex
schema path, without incurring too much overhead (vector transfer happens at batch level,
not it's merely reference transfer; no memory copy involved).

It would be great if you can make sort to handle complex schema path directly. However, I
have doubt about such proposal's performance benefit, until the real performance measurement
prove my suspicion is wrong. 
 

 

> Sort cannot directly access map members, causes a data copy
> -----------------------------------------------------------
>
>                 Key: DRILL-5384
>                 URL: https://issues.apache.org/jira/browse/DRILL-5384
>             Project: Apache Drill
>          Issue Type: Improvement
>    Affects Versions: 1.10.0
>            Reporter: Paul Rogers
>            Priority: Minor
>
> Suppose we have a JSON structure for "orders" like this:
> {code}
> { customer: { id: 10, name: "fred" },
>   order: { id: 20, product: "Frammis 1000" } }
> {code}
> Suppose I want to sort by customer.id. Today, Drill will project customer.id up to the
top level as a temporary, hidden field. Drill will copy the data from the customer.id vector
to this new temporary field. Drill then sorts on the temporary column, and uses another project
to remove the columns.
> Clearly, this work, but it has a cost:
> * Extra two project operators.
> * Extra memory copy.
> * Sort must buffer both the original and copied data. This can double memory use in the
worst case.
> All of this is done simply to avoid having to reference "customer.id" in the sort.
> But, as explained in DRILL-5376, maps are just nested tuples; there is no need to copy
the data, the data is already right there in a value vector. The problem is that Drill's map
implementation makes it hard for the generated code to get at the "customer.id" vector.
> This ticket asks to allow the sort to work directly with nested scalars to avoid the
overhead explained above. To do this:
> 1. Fix nested scalar access to allow the generated code to easily access a nested scalar.
> 2. Allow a sort key of the form "customer.id".
> 3. Modify the planner to generate such sort keys instead of the dual projects.
> The result will be a leaner, faster sort operation when sorting on scalars within a map.
>   



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