diff --git a/src/backend/executor/nodeHashjoin.c b/src/backend/executor/nodeHashjoin.c
index f2c885afbe..87d8f3766e 100644
--- a/src/backend/executor/nodeHashjoin.c
+++ b/src/backend/executor/nodeHashjoin.c
@@ -6,10 +6,78 @@
  * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- *
  * IDENTIFICATION
  *	  src/backend/executor/nodeHashjoin.c
  *
+ * NOTES:
+ *
+ * PARALLELISM
+ *
+ * Hash joins can participate in parallel queries in two ways: in
+ * non-parallel-aware mode, where each backend builds an identical hash table
+ * and then probes it with a partial outer relation, or parallel-aware mode
+ * where there is a shared hash table that all participants help to build.  A
+ * parallel-aware hash join can save time and space by dividing the work up
+ * and sharing the result, but has extra communication overheads.