Rename nodes/relation.h to nodes/pathnodes.h.

The old name of this file was never a very good indication of what it
was for.  Now that there's also access/relation.h, we have a potential
confusion hazard as well, so let's rename it to something more apropos.
Per discussion, "pathnodes.h" is reasonable, since a good fraction of
the file is Path node definitions.

While at it, tweak a couple of other headers that were gratuitously
importing relation.h into modules that don't need it.

Discussion: https://postgr.es/m/7719.1548688728@sss.pgh.pa.us

src/backend/nodes/README

@@ -36,8 +36,8 @@ FILES IN src/include/nodes/
 	nodes.h		- define node tags (NodeTag)
 	primnodes.h	- primitive nodes
 	parsenodes.h	- parse tree nodes
+	pathnodes.h	- path tree nodes and planner internal structures
 	plannodes.h	- plan tree nodes
-	relation.h	- planner internal nodes
 	execnodes.h	- executor nodes
 	memnodes.h	- memory nodes
 	pg_list.h	- generic list

src/backend/nodes/copyfuncs.c

@@ -24,8 +24,8 @@
 
 #include "miscadmin.h"
 #include "nodes/extensible.h"
+#include "nodes/pathnodes.h"
 #include "nodes/plannodes.h"
-#include "nodes/relation.h"
 #include "utils/datum.h"
 #include "utils/rel.h"
 
@@ -2196,7 +2196,7 @@ _copyOnConflictExpr(const OnConflictExpr *from)
 }
 
 /* ****************************************************************
- *						relation.h copy functions
+ *						pathnodes.h copy functions
  *
  * We don't support copying RelOptInfo, IndexOptInfo, or Path nodes.
  * There are some subsidiary structs that are useful to copy, though.

src/backend/nodes/equalfuncs.c

@@ -31,7 +31,7 @@
 
 #include "miscadmin.h"
 #include "nodes/extensible.h"
-#include "nodes/relation.h"
+#include "nodes/pathnodes.h"
 #include "utils/datum.h"
 
 
@@ -814,7 +814,7 @@ _equalOnConflictExpr(const OnConflictExpr *a, const OnConflictExpr *b)
 }
 
 /*
- * Stuff from relation.h
+ * Stuff from pathnodes.h
  */
 
 static bool

src/backend/nodes/nodeFuncs.c

@@ -20,7 +20,7 @@
 #include "nodes/makefuncs.h"
 #include "nodes/execnodes.h"
 #include "nodes/nodeFuncs.h"
-#include "nodes/relation.h"
+#include "nodes/pathnodes.h"
 #include "utils/builtins.h"
 #include "utils/lsyscache.h"
 

src/backend/nodes/outfuncs.c

@@ -32,8 +32,8 @@
 #include "lib/stringinfo.h"
 #include "miscadmin.h"
 #include "nodes/extensible.h"
+#include "nodes/pathnodes.h"
 #include "nodes/plannodes.h"
-#include "nodes/relation.h"
 #include "utils/datum.h"
 #include "utils/rel.h"
 
@@ -1676,7 +1676,7 @@ _outOnConflictExpr(StringInfo str, const OnConflictExpr *node)
 
 /*****************************************************************************
  *
- *	Stuff from relation.h.
+ *	Stuff from pathnodes.h.
  *
  *****************************************************************************/
 

src/backend/nodes/print.c

@@ -22,8 +22,8 @@
 #include "access/printtup.h"
 #include "lib/stringinfo.h"
 #include "nodes/nodeFuncs.h"
+#include "nodes/pathnodes.h"
 #include "nodes/print.h"
-#include "nodes/relation.h"
 #include "parser/parsetree.h"
 #include "utils/lsyscache.h"