tableam: Move heap specific logic from estimate_rel_size below tableam.

This just moves the table/matview[/toast] determination of relation
size to a callback, and uses a copy of the existing logic to implement
that callback for heap.

It probably would make sense to also move the index specific logic
into a callback, so the metapage handling (and probably more) can be
index specific. But that's a separate task.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

src/include/access/tableam.h

@@ -491,6 +491,22 @@ typedef struct TableAmRoutine
 										Snapshot snapshot,
 										struct ValidateIndexState *state);
 
+
+	/* ------------------------------------------------------------------------
+	 * Planner related functions.
+	 * ------------------------------------------------------------------------
+	 */
+
+	/*
+	 * See table_relation_estimate_size().
+	 *
+	 * While block oriented, it shouldn't be too hard to for an AM that
+	 * doesn't internally use blocks to convert into a usable representation.
+	 */
+	void		(*relation_estimate_size) (Relation rel, int32 *attr_widths,
+										   BlockNumber *pages, double *tuples,
+										   double *allvisfrac);
+
 } TableAmRoutine;
 
 
@@ -1286,6 +1302,25 @@ table_index_validate_scan(Relation heap_rel,
 }
 
 
+/* ----------------------------------------------------------------------------
+ * Planner related functionality
+ * ----------------------------------------------------------------------------
+ */
+
+/*
+ * Estimate the current size of the relation, as an AM specific workhorse for
+ * estimate_rel_size(). Look there for an explanation of the parameters.
+ */
+static inline void
+table_relation_estimate_size(Relation rel, int32 *attr_widths,
+							 BlockNumber *pages, double *tuples,
+							 double *allvisfrac)
+{
+	rel->rd_tableam->relation_estimate_size(rel, attr_widths, pages, tuples,
+											allvisfrac);
+}
+
+
 /* ----------------------------------------------------------------------------
  * Functions to make modifications a bit simpler.
  * ----------------------------------------------------------------------------

src/include/optimizer/plancat.h

@@ -33,6 +33,7 @@ extern List *infer_arbiter_indexes(PlannerInfo *root);
 extern void estimate_rel_size(Relation rel, int32 *attr_widths,
 				  BlockNumber *pages, double *tuples, double *allvisfrac);
 
+extern int32 get_rel_data_width(Relation rel, int32 *attr_widths);
 extern int32 get_relation_data_width(Oid relid, int32 *attr_widths);
 
 extern bool relation_excluded_by_constraints(PlannerInfo *root,