Add support for INSERT ... ON CONFLICT DO NOTHING/UPDATE.

The newly added ON CONFLICT clause allows to specify an alternative to
raising a unique or exclusion constraint violation error when inserting.
ON CONFLICT refers to constraints that can either be specified using a
inference clause (by specifying the columns of a unique constraint) or
by naming a unique or exclusion constraint.  DO NOTHING avoids the
constraint violation, without touching the pre-existing row.  DO UPDATE
SET ... [WHERE ...] updates the pre-existing tuple, and has access to
both the tuple proposed for insertion and the existing tuple; the
optional WHERE clause can be used to prevent an update from being
executed.  The UPDATE SET and WHERE clauses have access to the tuple
proposed for insertion using the "magic" EXCLUDED alias, and to the
pre-existing tuple using the table name or its alias.

This feature is often referred to as upsert.

This is implemented using a new infrastructure called "speculative
insertion". It is an optimistic variant of regular insertion that first
does a pre-check for existing tuples and then attempts an insert.  If a
violating tuple was inserted concurrently, the speculatively inserted
tuple is deleted and a new attempt is made.  If the pre-check finds a
matching tuple the alternative DO NOTHING or DO UPDATE action is taken.
If the insertion succeeds without detecting a conflict, the tuple is
deemed inserted.

To handle the possible ambiguity between the excluded alias and a table
named excluded, and for convenience with long relation names, INSERT
INTO now can alias its target table.

Bumps catversion as stored rules change.

Author: Peter Geoghegan, with significant contributions from Heikki
    Linnakangas and Andres Freund. Testing infrastructure by Jeff Janes.
Reviewed-By: Heikki Linnakangas, Andres Freund, Robert Haas, Simon Riggs,
    Dean Rasheed, Stephen Frost and many others.

src/include/nodes/primnodes.h

@@ -1143,6 +1143,22 @@ typedef struct CurrentOfExpr
 	int			cursor_param;	/* refcursor parameter number, or 0 */
 } CurrentOfExpr;
 
+/*
+ * InferenceElem - an element of a unique index inference specification
+ *
+ * This mostly matches the structure of IndexElems, but having a dedicated
+ * primnode allows for a clean separation between the use of index parameters
+ * by utility commands, and this node.
+ */
+typedef struct InferenceElem
+{
+	Expr		xpr;
+	Node	   *expr;				/* expression to infer from, or NULL */
+	Oid			infercollid;		/* OID of collation, or InvalidOid */
+	Oid			inferopfamily;		/* OID of att opfamily, or InvalidOid */
+	Oid			inferopcinputtype;	/* OID of att input type, or InvalidOid */
+} InferenceElem;
+
 /*--------------------
  * TargetEntry -
  *	   a target entry (used in query target lists)
@@ -1307,4 +1323,30 @@ typedef struct FromExpr
 	Node	   *quals;			/* qualifiers on join, if any */
 } FromExpr;
 
+/*----------
+ * OnConflictExpr - represents an ON CONFLICT DO ... expression
+ *
+ * The optimizer requires a list of inference elements, and optionally a WHERE
+ * clause to infer a unique index.  The unique index (or, occasionally,
+ * indexes) inferred are used to arbitrate whether or not the alternative ON
+ * CONFLICT path is taken.
+ *----------
+ */
+typedef struct OnConflictExpr
+{
+	NodeTag		type;
+	OnConflictAction action;	/* DO NOTHING or UPDATE? */
+
+	/* Arbiter */
+	List	   *arbiterElems;	/* unique index arbiter list (of InferenceElem's) */
+	Node	   *arbiterWhere;	/* unique index arbiter WHERE clause */
+	Oid			constraint;		/* pg_constraint OID for arbiter */
+
+	/* ON CONFLICT UPDATE */
+	List	   *onConflictSet;	/* List of ON CONFLICT SET TargetEntrys */
+	Node	   *onConflictWhere;/* qualifiers to restrict UPDATE to */
+	int			exclRelIndex;	/* RT index of 'excluded' relation */
+	List	   *exclRelTlist;	/* tlist of the EXCLUDED pseudo relation */
+} OnConflictExpr;
+
 #endif   /* PRIMNODES_H */