avamingli commented on code in PR #1311:
URL: https://github.com/apache/cloudberry/pull/1311#discussion_r2339100408
##########
src/backend/optimizer/path/allpaths.c:
##########
@@ -1839,8 +1839,38 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo
*rel,
* estimate.
*/
partial_rows = appendpath->path.rows;
- /* Add the path if subpath has not Motion.*/
- if (appendpath->path.parallel_safe &&
appendpath->path.motionHazard == false)
+
+ if (enable_parallel_append)
+ {
+ /* Add the path if subpath didn't encounter
motion hazard.*/
+ if (appendpath->path.parallel_safe &&
(appendpath->path.motionHazard == false))
+ add_partial_path(rel, (Path
*)appendpath);
+ else
+ {
+ /*
+ * CBDB_PARALLEL:
+ * When a parallel-aware Append is
dropped due to motion hazard,
+ * we attempt a second pass using
parallel-oblivious Append.
+ *
+ * This approach is feasible in CBDB
because:
+ * 1. All Motions in a parallel plan
handle tuples individually
+ * 2. Parallel Append might miss
executing slices containing Motions,
+ * whereas regular Append does not
have this problem
+ *
+ * This behavior is conceptually
similar to UPSTREAM's Append node
+ * with partial paths implementation.
+ */
+ appendpath = create_append_path(root,
rel, NIL, partial_subpaths,
+
NIL, NULL, parallel_workers,
+
false /*enable_parallel_append*/,
+
-1);
+ partial_rows = appendpath->path.rows;
Review Comment:
We try a parallel-oblivious approach for parallel append when
parallel-aware append is unsafe to use.
The previous partial_rows calculations were designed for a parallel-aware
append path. If the parallel-aware append path is deemed safe, we can directly
add the path without adjustments.
Otherwise, we need to correct the partial_rows values accordingly.
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