Support RIGHT JOIN and FULL OUTER JOIN in StandardJoin

[fupelaqu]

Problem

StandardJoin.validate() in sql/src/main/scala/app/softnetwork/elastic/sql/query/From.scala currently rejects all join types except InnerJoin and LeftJoin:

override def validate(): Either[String, Unit] = {
  for {
    _ <- joinType match {
      case Some(InnerJoin | LeftJoin) => Right(())
      case None                       => Right(()) // by default INNER JOIN
      case _ => Left(s"Standard JOIN $this requires an INNER (default) or LEFT JOIN type")
    }
    ...
  } yield ()
}

This means RIGHT JOIN and FULL OUTER JOIN are rejected at parse time, even though the downstream engines (DuckDB in federation, Elasticsearch for single-cluster) could support them.

Impact

• Federation queries using RIGHT JOIN or FULL OUTER JOIN fail with a parser error before reaching the query planner.
• Users must rewrite RIGHT JOIN as LEFT JOIN with swapped table order, which is unintuitive.

Proposed Fix

Extend StandardJoin.validate() to accept RightJoin and FullJoin (at minimum for federation mode where DuckDB handles the join).

Context

Discovered during Story 3.6 (End-to-End Federation Query Path) adversarial review.

🤖 Generated with Claude Code