diff --git a/contrib/pg_stat_statements/pg_stat_statements.c b/contrib/pg_stat_statements/pg_stat_statements.c
index b04b4d6ce1..829ee69f51 100644
--- a/contrib/pg_stat_statements/pg_stat_statements.c
+++ b/contrib/pg_stat_statements/pg_stat_statements.c
@@ -124,7 +124,10 @@ typedef enum pgssVersion
 
 /*
  * Hashtable key that defines the identity of a hashtable entry.  We separate
- * queries by user and by database even if they are otherwise identical.
+ * queries by user and by database even if they are otherwise identical.  Be
+ * careful when adding new fields, tag_hash() is used to compute the hash key,
+ * so we rely on the fact that no padding bit is present in this structure.
+ * Otherwise, we'd have to zero the key variable in pgss_store.
  */
 typedef struct pgssHashKey
 {