Re: [PR] [CALCITE-7279] Add subquery wrappers for nested JOINs in ClickHouse dialect [calcite]

["Dwrite (via GitHub)" <gi...@apache.org>]

Dwrite commented on code in PR #4692:
URL: https://github.com/apache/calcite/pull/4692#discussion_r2637132295


##########
core/src/main/java/org/apache/calcite/sql/dialect/ClickHouseSqlDialect.java:
##########
@@ -404,4 +411,37 @@ private static void unparseFloor(SqlWriter writer, SqlCall call) {
     call.operand(0).unparse(writer, 0, 0);
     writer.endList(frame);
   }
+
+  @Override public boolean mustWrapNestedJoin(RelNode rel) {
+    if (!(rel instanceof Join)) {
+      return false;
+    }
+    Join join = (Join) rel;
+
+    // ClickHouse primarily requires wrapping the right side of a JOIN
+    // when it contains a nested JOIN
+    return containsJoinRecursive(join.getRight());
+  }
+
+  /**
+   * Recursively checks if a RelNode contains a JOIN, looking through
+   * transparent single-input operators.
+   */
+  private static boolean containsJoinRecursive(RelNode rel) {

Review Comment:
   Good question.
   
   A visitor would indeed be the right tool if we needed a full or deep traversal
   of the relational tree. In this case, however, we intentionally keep the check
   shallow.
   
   For ClickHouse, the restriction only applies when a JOIN appears directly as
   the right (or in some cases left) input of another JOIN, possibly wrapped by a
   small number of transparent operators (e.g. Project / Filter / Sort).
   We do not want to detect JOINs arbitrarily deep in the subtree.
   
   Using a visitor here would traverse deeper than necessary and could lead to
   over-wrapping in cases where the nested JOIN is not syntactically exposed at the
   JOIN boundary.
   
   For this reason, a small structural check is used instead of a general visitor.
   I’ve added comments to clarify this intent.



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