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https://issues.apache.org/jira/browse/DRILL-6373?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16509147#comment-16509147
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ASF GitHub Bot commented on DRILL-6373:
---------------------------------------
paul-rogers commented on issue #1244: DRILL-6373: Refactor Result Set Loader
for Union, List support
URL: https://github.com/apache/drill/pull/1244#issuecomment-396460011
@vrozov, this is what I meant by the code only working well for a flat
vector. Maps have a `MaterializedField` for their own metadata. That metadata
contains a list of the child metadata. So:
```
m:Map, {a:Int, b:Varchar}
. a:INT
. b:Varchar
```
This means, as we add `a`, then, `b` to map `m`, we must mutate the
`MaterializedField` for `m` to add the child fields. This completely breaks the
idea that a `MaterializedField` is always immutable. This is a place where a
design goal (immutable `MateraializedField`) collides with implementation
(readers add to maps as they find new fields).
So. We could make a clone on each modification and problem solved, right? As
in many places in Drill, the solution is not so simple. The above is the easy
case. What about a nested map:
```
m1:Map, {m2:Map {a:Int, b:Varchar}}
. m2:Map, {a:Int, b:Varchar}
. . a:INT
. . b:Varchar
```
Now when we add `a`, we have to update both the `m1` and `m2`
`MaterializedField`s. If we clone the MaterializedField for `m2`, then the old
version in `m1` will get out of sync. The result will be:
```
m1:Map, {m2:Map {}}
. m2:Map, {a:Int, b:Varchar}
. . a:INT
. . b:Varchar
```
Code that depends on accurate schema information then breaks.
So, we're left with a choice: clone and have a corrupt schema, or make
`MaterializedField` mutable.
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> Refactor the Result Set Loader to prepare for Union, List support
> -----------------------------------------------------------------
>
> Key: DRILL-6373
> URL: https://issues.apache.org/jira/browse/DRILL-6373
> Project: Apache Drill
> Issue Type: Improvement
> Affects Versions: 1.13.0
> Reporter: Paul Rogers
> Assignee: Paul Rogers
> Priority: Major
> Fix For: 1.14.0
>
>
> As the next step in merging the "batch sizing" enhancements, refactor the
> {{ResultSetLoader}} and related classes to prepare for Union and List
> support. This fix follows the refactoring of the column accessors for the
> same purpose. Actual Union and List support is to follow in a separate PR.
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