tree 8ab02373d35c7f0054c95eafc51220f8b1baf113
parent abce61f3fda73e865a80e9c38bf9ca471a6a5db8
author Wenchen Fan <wenchen@databricks.com> 1627629992 -0700
committer Liang-Chi Hsieh <viirya@gmail.com> 1627629992 -0700

[SPARK-34952][SQL][FOLLOWUP] Simplify JDBC aggregate pushdown

### What changes were proposed in this pull request?

This is a followup of https://github.com/apache/spark/pull/33352 , to simplify the JDBC aggregate pushdown:
1. We should get the schema of the aggregate query by asking the JDBC server, instead of calculating it by ourselves. This can simplify the code a lot, and is also more robust: the data type of SUM may vary in different databases, it's fragile to assume they are always the same as Spark.
2. because of 1, now we can remove the `dataType` property from the public `Sum` expression.

This PR also contains some small improvements:
1. Spark should deduplicate the aggregate expressions before pushing them down.
2. Improve the `toString` of public aggregate expressions to make them more SQL.

### Why are the changes needed?

code and API simplification

### Does this PR introduce _any_ user-facing change?

this API is not released yet.

### How was this patch tested?

existing tests

Closes #33579 from cloud-fan/dsv2.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Liang-Chi Hsieh <viirya@gmail.com>