vldpyatkov commented on code in PR #5370:
URL: https://github.com/apache/calcite/pull/5370#discussion_r4530363323
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core/src/main/java/org/apache/calcite/sql/validate/RecursiveWithRewriter.java:
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@@ -110,25 +114,98 @@ static SqlWithItem rewrite(SqlValidatorImpl validator,
SqlValidatorScope scope,
* The added WHERE condition stops expanding rows already marked as cycles.
* The recursive output keys are taken after the user's projection. A row
* closing a cycle is therefore returned with its extended path, but cannot
- * feed the next recursive step. The original UNION [ALL] is retained.
- * The returned WITH item declares the two additional columns and no longer
- * has a CYCLE clause. */
+ * feed the next recursive step.
+ *
+ * <p>For {@code SEARCH DEPTH FIRST BY k1, k2 SET searchOrder}, the
+ * transformation is schematically:
+ *
+ * <pre>{@code
+ * seed output:
+ * original columns,
+ * ARRAY[ROW(k1, k2)] AS searchOrder
+ * recursive branch:
+ * SELECT original output columns,
+ * append(previousOrder, ROW(k1, k2)) AS searchOrder
+ * FROM (
+ * SELECT original projection, previous.searchOrder AS previousOrder
+ * FROM original sources
+ * WHERE original condition
+ * ) AS step
+ * }</pre>
+ *
+ * <p>The generated array contains the key tuples along the path, including
+ * the current row. Lexicographic ordering of these arrays places a parent
+ * before its descendants. For distinct sibling keys, a node's descendants
+ * precede the next sibling.
+ * The keys within each tuple follow the order of the BY list. Even a single
+ * BY column is wrapped in ROW, so the generated column is always an array
+ * of rows. The internal {@link SqlInternalOperators#RECURSIVE_PATH_APPEND}
+ * operator implements append.
+ *
+ * <p>For {@code SEARCH BREADTH FIRST BY k1, k2 SET searchOrder}, the
+ * transformation is schematically:
+ *
+ * <pre>{@code
+ * seed output:
+ * original columns,
+ * ROW(0, k1, k2) AS searchOrder
+ * recursive branch:
+ * SELECT original output columns,
+ * ROW(previousOrder.$DEPTH + 1, k1, k2) AS searchOrder
+ * FROM (
+ * SELECT original projection, previous.searchOrder AS previousOrder
+ * FROM original sources
+ * WHERE original condition
+ * ) AS step
+ * }</pre>
+ *
+ * <p>Here {@code $DEPTH} denotes the first field of the generated row. It is
Review Comment:
Test added RecursiveCteTest#testSearchBreadthFirstDepthNameCollision.
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