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diff --git a/src/include/nodes/extensible.h b/src/include/nodes/extensible.h
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+/*-------------------------------------------------------------------------
+ *
+ * extensible.h
+ *	  Definitions for extensible nodes and custom scans
+ *
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/nodes/extensible.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef EXTENSIBLE_H
+#define EXTENSIBLE_H
+
+#include "access/parallel.h"
+#include "commands/explain.h"
+#include "nodes/execnodes.h"
+#include "nodes/pathnodes.h"
+#include "nodes/plannodes.h"
+
+/* maximum length of an extensible node identifier */
+#define EXTNODENAME_MAX_LEN					64
+
+/*
+ * An extensible node is a new type of node defined by an extension.  The
+ * type is always T_ExtensibleNode, while the extnodename identifies the
+ * specific type of node.  extnodename can be looked up to find the
+ * ExtensibleNodeMethods for this node type.
+ */
+typedef struct ExtensibleNode
+{
+	NodeTag		type;
+	const char *extnodename;	/* identifier of ExtensibleNodeMethods */
+} ExtensibleNode;
+
+/*
+ * node_size is the size of an extensible node of this type in bytes.
+ *
+ * nodeCopy is a function which performs a deep copy from oldnode to newnode.
+ * It does not need to copy type or extnodename, which are copied by the
+ * core system.
+ *
+ * nodeEqual is a function which performs a deep equality comparison between
+ * a and b and returns true or false accordingly.  It does not need to compare
+ * type or extnodename, which are compared by the core system.
+ *
+ * nodeOut is a serialization function for the node type.  It should use the
+ * output conventions typical for outfuncs.c.  It does not need to output
+ * type or extnodename; the core system handles those.
+ *
+ * nodeRead is a deserialization function for the node type.  It does not need
+ * to read type or extnodename; the core system handles those.  It should fetch
+ * the next token using pg_strtok() from the current input stream, and then
+ * reconstruct the private fields according to the manner in readfuncs.c.
+ *
+ * All callbacks are mandatory.
+ */
+typedef struct ExtensibleNodeMethods
+{
+	const char *extnodename;
+	Size		node_size;
+	void		(*nodeCopy) (struct ExtensibleNode *newnode,
+							 const struct ExtensibleNode *oldnode);
+	bool		(*nodeEqual) (const struct ExtensibleNode *a,
+							  const struct ExtensibleNode *b);
+	void		(*nodeOut) (struct StringInfoData *str,
+							const struct ExtensibleNode *node);
+	void		(*nodeRead) (struct ExtensibleNode *node);
+} ExtensibleNodeMethods;
+
+extern void RegisterExtensibleNodeMethods(const ExtensibleNodeMethods *method);
+extern const ExtensibleNodeMethods *GetExtensibleNodeMethods(const char *name,
+															 bool missing_ok);
+
+/*
+ * Flags for custom paths, indicating what capabilities the resulting scan
+ * will have.
+ */
+#define CUSTOMPATH_SUPPORT_BACKWARD_SCAN	0x0001
+#define CUSTOMPATH_SUPPORT_MARK_RESTORE		0x0002
+
+/*
+ * Custom path methods.  Mostly, we just need to know how to convert a
+ * CustomPath to a plan.
+ */
+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,
+									List *custom_plans);
+	struct List *(*ReparameterizeCustomPathByChild) (PlannerInfo *root,
+													 List *custom_private,
+													 RelOptInfo *child_rel);
+}			CustomPathMethods;
+
+/*
+ * Custom scan.  Here again, there's not much to do: we need to be able to
+ * generate a ScanState corresponding to the scan.
+ */
+typedef struct CustomScanMethods
+{
+	const char *CustomName;
+
+	/* Create execution state (CustomScanState) from a CustomScan plan node */
+	Node	   *(*CreateCustomScanState) (CustomScan *cscan);
+} CustomScanMethods;
+
+/*
+ * Execution-time methods for a CustomScanState.  This is more complex than
+ * what we need for a custom path or scan.
+ */
+typedef struct CustomExecMethods
+{
+	const char *CustomName;
+
+	/* Required executor methods */
+	void		(*BeginCustomScan) (CustomScanState *node,
+									EState *estate,
+									int eflags);
+	TupleTableSlot *(*ExecCustomScan) (CustomScanState *node);
+	void		(*EndCustomScan) (CustomScanState *node);
+	void		(*ReScanCustomScan) (CustomScanState *node);
+
+	/* Optional methods: needed if mark/restore is supported */
+	void		(*MarkPosCustomScan) (CustomScanState *node);
+	void		(*RestrPosCustomScan) (CustomScanState *node);
+
+	/* Optional methods: needed if parallel execution is supported */
+	Size		(*EstimateDSMCustomScan) (CustomScanState *node,
+										  ParallelContext *pcxt);
+	void		(*InitializeDSMCustomScan) (CustomScanState *node,
+											ParallelContext *pcxt,
+											void *coordinate);
+	void		(*ReInitializeDSMCustomScan) (CustomScanState *node,
+											  ParallelContext *pcxt,
+											  void *coordinate);
+	void		(*InitializeWorkerCustomScan) (CustomScanState *node,
+											   shm_toc *toc,
+											   void *coordinate);
+	void		(*ShutdownCustomScan) (CustomScanState *node);
+
+	/* Optional: print additional information in EXPLAIN */
+	void		(*ExplainCustomScan) (CustomScanState *node,
+									  List *ancestors,
+									  ExplainState *es);
+} CustomExecMethods;
+
+extern void RegisterCustomScanMethods(const CustomScanMethods *methods);
+extern const CustomScanMethods *GetCustomScanMethods(const char *CustomName,
+													 bool missing_ok);
+
+#endif							/* EXTENSIBLE_H */