tableam: New callback relation_fetch_toast_slice.

Instead of always calling heap_fetch_toast_slice during detoasting,
invoke a table AM callback which, when the toast table is a heap
table, will be heap_fetch_toast_slice.

This makes it possible for a table AM other than heap to be used
as a TOAST table. It also completes the series of commits intended
to improve the interaction of tableam with TOAST that began with
commit 8b94dab06617ef80a0901ab103ebd8754427ef5a; detoast.c is
now, hopefully, fully AM-independent.

Patch by me, reviewed by Andres Freund and Peter Eisentraut.

Discussion: http://postgr.es/m/CA+TgmoZv-=2iWM4jcw5ZhJeL18HF96+W1yJeYrnGMYdkFFnEpQ@mail.gmail.com

src/include/access/heaptoast.h

@@ -136,4 +136,14 @@ extern HeapTuple toast_build_flattened_tuple(TupleDesc tupleDesc,
 											 Datum *values,
 											 bool *isnull);
 
+/* ----------
+ * heap_fetch_toast_slice
+ *
+ *	Fetch a slice from a toast value stored in a heap table.
+ * ----------
+ */
+extern void heap_fetch_toast_slice(Relation toastrel, Oid valueid,
+								   int32 attrsize, int32 sliceoffset,
+								   int32 slicelength, struct varlena *result);
+
 #endif							/* HEAPTOAST_H */

src/include/access/tableam.h

@@ -588,6 +588,17 @@ typedef struct TableAmRoutine
 	 */
 	Oid		    (*relation_toast_am) (Relation rel);
 
+	/*
+	 * This callback is invoked when detoasting a value stored in a toast
+	 * table implemented by this AM.  See table_relation_fetch_toast_slice()
+	 * for more details.
+	 */
+	void		(*relation_fetch_toast_slice) (Relation toastrel, Oid valueid,
+											   int32 attrsize,
+											   int32 sliceoffset,
+											   int32 slicelength,
+											   struct varlena *result);
+
 
 	/* ------------------------------------------------------------------------
 	 * Planner related functions.
@@ -1620,6 +1631,41 @@ table_relation_toast_am(Relation rel)
 	return rel->rd_tableam->relation_toast_am(rel);
 }
 
+/*
+ * Fetch all or part of a TOAST value from a TOAST table.
+ *
+ * If this AM is never used to implement a TOAST table, then this callback
+ * is not needed. But, if toasted values are ever stored in a table of this
+ * type, then you will need this callback.
+ *
+ * toastrel is the relation in which the toasted value is stored.
+ *
+ * valueid identifes which toast value is to be fetched. For the heap,
+ * this corresponds to the values stored in the chunk_id column.
+ *
+ * attrsize is the total size of the toast value to be fetched.
+ *
+ * sliceoffset is the offset within the toast value of the first byte that
+ * should be fetched.
+ *
+ * slicelength is the number of bytes from the toast value that should be
+ * fetched.
+ *
+ * result is caller-allocated space into which the fetched bytes should be
+ * stored.
+ */
+static inline void
+table_relation_fetch_toast_slice(Relation toastrel, Oid valueid,
+								 int32 attrsize, int32 sliceoffset,
+								 int32 slicelength, struct varlena *result)
+{
+	return toastrel->rd_tableam->relation_fetch_toast_slice(toastrel, valueid,
+															attrsize,
+															sliceoffset,
+															slicelength,
+															result);
+}
+
 
 /* ----------------------------------------------------------------------------
  * Planner related functionality