add more comments for RowMarks fixes etc

Author: funbringer

src/include/compat/relation_tags.h

@@ -2,7 +2,13 @@
  *
  * relation_tags.h
  *		Attach custom (Key, Value) pairs to an arbitrary RangeTblEntry
- *		NOTE: implementations for vanilla and PostgresPro differ
+ *
+ *		NOTE: implementations for vanilla and PostgresPro differ,
+ *		which means that subquery pull-up might break the bond
+ *		between a RangeTblEntry and the corresponding KVPs.
+ *
+ *		This subsystem was meant to replace the broken 'inh' flag
+ *		(see get_rel_parenthood_status() for more details).
  *
  * Copyright (c) 2017, Postgres Professional
  *

src/include/compat/rowmarks_fix.h

@@ -29,7 +29,16 @@ void append_tle_for_rowmark(PlannerInfo *root, PlanRowMark *rc);
 
 #else
 
-#define LEGACY_ROWMARKS_95 /* NOTE: can't fix 9.5, see PlannerInfo->processed_tlist */
+/*
+ * Starting from 9.6, it's possible to append junk
+ * tableoid columns using the PlannerInfo->processed_tlist.
+ * This is absolutely crucial for UPDATE and DELETE queries,
+ * so we had to add some special fixes for 9.5:
+ *
+ *		1) provide legacy code for RowMarks (tableoids);
+ *		2) disable dangerous UPDATE & DELETE optimizations.
+ */
+#define LEGACY_ROWMARKS_95
 
 #define append_tle_for_rowmark(root, rc)	( (void) true )
 

src/planner_tree_modification.c

@@ -44,14 +44,16 @@
 
 typedef struct
 {
-	/* bool has_parent_CMD_TYPE_query; */
+	/* Do we have a parent CmdType query? */
 	bool			TRANSFORM_CONTEXT_FIELD(SELECT),
 					TRANSFORM_CONTEXT_FIELD(INSERT),
 					TRANSFORM_CONTEXT_FIELD(UPDATE),
 					TRANSFORM_CONTEXT_FIELD(DELETE);
 
-	/* params for handle_modification_query() */
+	/* Parameters for handle_modification_query() */
 	ParamListInfo	query_params;
+
+	/* SubLink that might contain an examined query */
 	SubLink		   *parent_sublink;
 } transform_query_cxt;