3 files changed, 38 insertions, 26 deletions

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 2bffa4be6cd..41b13b2b670 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1503,9 +1503,16 @@ typedef struct ForeignScanState
 } ForeignScanState;
 
 /* ----------------
- * CustomScanState information
+ *	 CustomScanState information
  *
  *		CustomScan nodes are used to execute custom code within executor.
+ *
+ * Core code must avoid assuming that the CustomScanState is only as large as
+ * the structure declared here; providers are allowed to make it the first
+ * element in a larger structure, and typically would need to do so.  The
+ * struct is actually allocated by the CreateCustomScanState method associated
+ * with the plan node.  Any additional fields can be initialized there, or in
+ * the BeginCustomScan method.
  * ----------------
  */
 struct ExplainState;			/* avoid including explain.h here */
@@ -1515,7 +1522,7 @@ typedef struct CustomExecMethods
 {
 	const char *CustomName;
 
-	/* EXECUTOR methods */
+	/* Executor methods: mark/restore are optional, the rest are required */
 	void		(*BeginCustomScan) (struct CustomScanState *node,
 												EState *estate,
 												int eflags);
@@ -1525,13 +1532,10 @@ typedef struct CustomExecMethods
 	void		(*MarkPosCustomScan) (struct CustomScanState *node);
 	void		(*RestrPosCustomScan) (struct CustomScanState *node);
 
-	/* EXPLAIN support */
+	/* Optional: print additional information in EXPLAIN */
 	void		(*ExplainCustomScan) (struct CustomScanState *node,
 												  List *ancestors,
 												  struct ExplainState *es);
-	Node	   *(*GetSpecialCustomVar) (struct CustomScanState *node,
-													Var *varnode,
-													PlanState **child_ps);
 } CustomExecMethods;
 
 typedef struct CustomScanState
diff --git a/src/include/nodes/plannodes.h b/src/include/nodes/plannodes.h
index dd300b1a191..7f9eaf0df26 100644
--- a/src/include/nodes/plannodes.h
+++ b/src/include/nodes/plannodes.h
@@ -486,32 +486,32 @@ typedef struct ForeignScan
 
 /* ----------------
  *	   CustomScan node
+ *
+ * The comments for ForeignScan's fdw_exprs and fdw_private fields apply
+ * equally to custom_exprs and custom_private.  Note that since Plan trees
+ * can be copied, custom scan providers *must* fit all plan data they need
+ * into those fields; embedding CustomScan in a larger struct will not work.
  * ----------------
  */
-struct PlannerInfo;				/* avoid including relation.h here */
 struct CustomScan;
 
 typedef struct CustomScanMethods
 {
 	const char *CustomName;
 
-	void		(*SetCustomScanRef) (struct PlannerInfo *root,
-												 struct CustomScan *cscan,
-												 int rtoffset);
-	void		(*FinalizeCustomScan) (struct PlannerInfo *root,
-												   struct CustomScan *cscan,
-												bool (*finalize_primnode) (),
-												   void *finalize_context);
+	/* Create execution state (CustomScanState) from a CustomScan plan node */
 	Node	   *(*CreateCustomScanState) (struct CustomScan *cscan);
+	/* Optional: print custom_xxx fields in some special way */
 	void		(*TextOutCustomScan) (StringInfo str,
 											  const struct CustomScan *node);
-	struct CustomScan *(*CopyCustomScan) (const struct CustomScan *from);
 } CustomScanMethods;
 
 typedef struct CustomScan
 {
 	Scan		scan;
 	uint32		flags;			/* mask of CUSTOMPATH_* flags, see relation.h */
+	List	   *custom_exprs;	/* expressions that custom code may evaluate */
+	List	   *custom_private; /* private data for custom code */
 	const CustomScanMethods *methods;
 } CustomScan;
 
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index bd5e8ce5c13..810b9c8893b 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -885,17 +885,22 @@ typedef struct ForeignPath
 } ForeignPath;
 
 /*
- * CustomPath represents a scan by some out-of-core extension.
- *
- * We provide a set of hooks here - which the provider must take care to
- * set up correctly - to allow extensions to supply their own methods of
- * scanning a relation.  For example, a provider might provide GPU
- * acceleration, a cache-based scan, or some other kind of logic we haven't
- * dreamed up yet.
- *
- * Core code should avoid assuming that the CustomPath is only as large as
- * the structure declared here; providers are expected to make it the first
- * element in a larger structure.
+ * CustomPath represents a table scan done by some out-of-core extension.
+ *
+ * We provide a set of hooks here - which the provider must take care to set
+ * up correctly - to allow extensions to supply their own methods of scanning
+ * a relation.  For example, a provider might provide GPU acceleration, a
+ * cache-based scan, or some other kind of logic we haven't dreamed up yet.
+ *
+ * CustomPaths can be injected into the planning process for a relation by
+ * set_rel_pathlist_hook functions.
+ *
+ * Core code must avoid assuming that the CustomPath is only as large as
+ * the structure declared here; providers are allowed to make it the first
+ * element in a larger structure.  (Since the planner never copies Paths,
+ * this doesn't add any complication.)  However, for consistency with the
+ * FDW case, we provide a "custom_private" field in CustomPath; providers
+ * may prefer to use that rather than define another struct type.
  */
 struct CustomPath;
 
@@ -906,11 +911,13 @@ typedef struct CustomPathMethods
 {
 	const char *CustomName;
 
+	/* Convert Path to a Plan */
 	struct Plan *(*PlanCustomPath) (PlannerInfo *root,
 												RelOptInfo *rel,
 												struct CustomPath *best_path,
 												List *tlist,
 												List *clauses);
+	/* Optional: print additional fields besides "private" */
 	void		(*TextOutCustomPath) (StringInfo str,
 											  const struct CustomPath *node);
 } CustomPathMethods;
@@ -919,6 +926,7 @@ typedef struct CustomPath
 {
 	Path		path;
 	uint32		flags;			/* mask of CUSTOMPATH_* flags, see above */
+	List	   *custom_private;
 	const CustomPathMethods *methods;
 } CustomPath;