[SPARK-49565][SQL] Improve auto-generated expression aliases with pipe SQL operators

[dtenedor]

What changes were proposed in this pull request?

This RP improves auto-generated expression aliases with pipe SQL operators.

For example, consider the pipe SQL syntax query:

table t
|> extend 1

Previously, the analyzed plan was:

Project [x#x, y#x, pipeexpression(1, false, EXTEND) AS pipeexpression(1)#x]
+- SubqueryAlias spark_catalog.default.t
   +- Relation spark_catalog.default.t[x#x,y#x] csv

After this PR, it is:

Project [x#x, y#x, pipeexpression(1, false, EXTEND) AS 1#x]
+- SubqueryAlias spark_catalog.default.t
   +- Relation spark_catalog.default.t[x#x,y#x] csv

Note that the output aliases visible in the resulting DataFrame for the query derive from the AS <alias> part of the analyzed plans shown.

Why are the changes needed?

This improves the user experience with pipe SQL syntax.

Does this PR introduce any user-facing change?

Yes, see above.

How was this patch tested?

Existing golden file tests update to show the improved aliases.

Was this patch authored or co-authored using generative AI tooling?

No.

[dtenedor]

cc @cloud-fan @gengliangwang

[cloud-fan]

Shall we make AliasResolution to properly support PipeExpression?

[dtenedor]

Shall we make AliasResolution to properly support PipeExpression?

@cloud-fan I tried this, but the PipeExpression needs to check whether its child expression tree contains any aggregate functions, and this needs its child tree to be fully resolved first (which may not have happened yet by the time we run the ResolveAliases rule).

[cloud-fan]

@dtenedor after a few iterations, ResolveAliases should see PipeExpression with its child resolved, right? The thing is that we should have consistent auto alias generation behavior between normal and pipe SQL syntaxes, e.g.

      case ne: NamedExpression => ne
      case go @ GeneratorOuter(g: Generator) if g.resolved => MultiAlias(go, Nil)
      case e if !e.resolved => u
      case g: Generator => MultiAlias(g, Nil)
      case c @ Cast(ne: NamedExpression, _, _, _) => Alias(c, ne.name)()
      case e: ExtractValue if extractOnly(e) => Alias(e, toPrettySQL(e))()