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+/*-------------------------------------------------------------------------
+ *
+ * supportnodes.h
+ *	  Definitions for planner support functions.
+ *
+ * This file defines the API for "planner support functions", which
+ * are SQL functions (normally written in C) that can be attached to
+ * another "target" function to give the system additional knowledge
+ * about the target function.  All the current capabilities have to do
+ * with planning queries that use the target function, though it is
+ * possible that future extensions will add functionality to be invoked
+ * by the parser or executor.
+ *
+ * A support function must have the SQL signature
+ *		supportfn(internal) returns internal
+ * The argument is a pointer to one of the Node types defined in this file.
+ * The result is usually also a Node pointer, though its type depends on
+ * which capability is being invoked.  In all cases, a NULL pointer result
+ * (that's PG_RETURN_POINTER(NULL), not PG_RETURN_NULL()) indicates that
+ * the support function cannot do anything useful for the given request.
+ * Support functions must return a NULL pointer, not fail, if they do not
+ * recognize the request node type or cannot handle the given case; this
+ * allows for future extensions of the set of request cases.
+ *
+ *
+ * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/nodes/supportnodes.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef SUPPORTNODES_H
+#define SUPPORTNODES_H
+
+#include "nodes/primnodes.h"
+
+struct PlannerInfo;				/* avoid including relation.h here */
+
+
+/*
+ * The Simplify request allows the support function to perform plan-time
+ * simplification of a call to its target function.  For example, a varchar
+ * length coercion that does not decrease the allowed length of its argument
+ * could be replaced by a RelabelType node, or "x + 0" could be replaced by
+ * "x".  This is invoked during the planner's constant-folding pass, so the
+ * function's arguments can be presumed already simplified.
+ *
+ * The planner's PlannerInfo "root" is typically not needed, but can be
+ * consulted if it's necessary to obtain info about Vars present in
+ * the given node tree.  Beware that root could be NULL in some usages.
+ *
+ * "fcall" will be a FuncExpr invoking the support function's target
+ * function.  (This is true even if the original parsetree node was an
+ * operator call; a FuncExpr is synthesized for this purpose.)
+ *
+ * The result should be a semantically-equivalent transformed node tree,
+ * or NULL if no simplification could be performed.  Do *not* return or
+ * modify *fcall, as it isn't really a separately allocated Node.  But
+ * it's okay to use fcall->args, or parts of it, in the result tree.
+ */
+typedef struct SupportRequestSimplify
+{
+	NodeTag		type;
+
+	struct PlannerInfo *root;	/* Planner's infrastructure */
+	FuncExpr   *fcall;			/* Function call to be simplified */
+} SupportRequestSimplify;
+
+#endif							/* SUPPORTNODES_H */