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From "Paul Rogers (JIRA)" <j...@apache.org>
Subject [jira] [Created] (DRILL-5384) Sort cannot directly access map members, causes a data copy
Date Fri, 24 Mar 2017 23:48:42 GMT
Paul Rogers created DRILL-5384:

             Summary: 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:

{ customer: { id: 10, name: "fred" },
  order: { id: 20, product: "Frammis 1000" } }

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

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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