[SPARK-26837][SQL] Pruning nested fields from object serializers #663
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## What changes were proposed in this pull request?
In SPARK-26619, we make change to prune unnecessary individual serializers when serializing objects. This is extension to SPARK-26619. We can further prune nested fields from object serializers if they are not used.
For example, in following query, we only use one field in a struct column:
```scala
val data = Seq((("a", 1), 1), (("b", 2), 2), (("c", 3), 3))
val df = data.toDS().map(t => (t._1, t._2 + 1)).select("_1._1")
```
So, instead of having a serializer to create a two fields struct, we can prune unnecessary field from it. This is what this PR proposes to do.
In order to make this change conservative and safer, a SQL config is added to control it. It is disabled by default.
TODO: Support to prune nested fields inside MapType's key and value.
## How was this patch tested?
Added tests.
Closes apache#23740 from viirya/nested-pruning-serializer-2.
Authored-by: Liang-Chi Hsieh <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>
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Proposed change
Picking up SPARK-26837 via apache#23740.
Bluf is that this will speed-up serialization by ignoring unused nested fields. Example provided was this
which would only select a, b, and c.
If I understand correctly, we already prune serialization for non-nested fields (the outer 1, 2, and 3) because we took SPARK-26619. After this we'll also prune serializers for the inner 1, 2, and 3.
How was this patch tested?
Upstream added tests.