diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index 6584a9cb8da..132e9466450 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -1962,6 +1962,31 @@ heapam_scan_get_blocks_done(HeapScanDesc hscan)
 }
 
 
+/* ------------------------------------------------------------------------
+ * Miscellaneous callbacks for the heap AM
+ * ------------------------------------------------------------------------
+ */
+
+static uint64
+heapam_relation_size(Relation rel, ForkNumber forkNumber)
+{
+	uint64		nblocks = 0;
+
+	/* Open it at the smgr level if not already done */
+	RelationOpenSmgr(rel);
+
+	/* InvalidForkNumber indicates the size of all forks */
+	if (forkNumber == InvalidForkNumber)
+	{
+		for (int i = 0; i < MAX_FORKNUM; i++)
+			nblocks += smgrnblocks(rel->rd_smgr, i);
+	}
+	else
+		nblocks = smgrnblocks(rel->rd_smgr, forkNumber);
+
+	return nblocks * BLCKSZ;
+}
+
 
 /* ------------------------------------------------------------------------
  * Planner related callbacks for the heap AM
@@ -2543,6 +2568,8 @@ static const TableAmRoutine heapam_methods = {
 	.index_build_range_scan = heapam_index_build_range_scan,
 	.index_validate_scan = heapam_index_validate_scan,
 
+	.relation_size = heapam_relation_size,
+
 	.relation_estimate_size = heapam_estimate_rel_size,
 
 	.scan_bitmap_next_block = heapam_scan_bitmap_next_block,
diff --git a/src/backend/access/table/tableamapi.c b/src/backend/access/table/tableamapi.c
index bfd713f3af1..2b632e002c4 100644
--- a/src/backend/access/table/tableamapi.c
+++ b/src/backend/access/table/tableamapi.c
@@ -86,6 +86,9 @@ GetTableAmRoutine(Oid amhandler)
 	Assert(routine->scan_analyze_next_tuple != NULL);
 	Assert(routine->index_build_range_scan != NULL);
 	Assert(routine->index_validate_scan != NULL);
+
+	Assert(routine->relation_size != NULL);
+
 	Assert(routine->relation_estimate_size != NULL);
 
 	/* optional, but one callback implies presence of hte other */
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 887023fc8a5..10ef0de78b4 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -33,6 +33,7 @@
 #include <sys/file.h>
 #include <unistd.h>
 
+#include "access/tableam.h"
 #include "access/xlog.h"
 #include "catalog/catalog.h"
 #include "catalog/storage.h"
@@ -2789,14 +2790,50 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln)
 /*
  * RelationGetNumberOfBlocksInFork
  *		Determines the current number of pages in the specified relation fork.
+ *
+ * Note that the accuracy of the result will depend on the details of the
+ * relation's storage. For all builtin AMs it'll be accurate, but for external
+ * AMs it might not perfectly be.
  */
 BlockNumber
 RelationGetNumberOfBlocksInFork(Relation relation, ForkNumber forkNum)
 {
-	/* Open it at the smgr level if not already done */
-	RelationOpenSmgr(relation);
+	switch (relation->rd_rel->relkind)
+	{
+		case RELKIND_SEQUENCE:
+		case RELKIND_INDEX:
+		case RELKIND_PARTITIONED_INDEX:
+			/* Open it at the smgr level if not already done */
+			RelationOpenSmgr(relation);
 
-	return smgrnblocks(relation->rd_smgr, forkNum);
+			return smgrnblocks(relation->rd_smgr, forkNum);
+
+		case RELKIND_RELATION:
+		case RELKIND_TOASTVALUE:
+		case RELKIND_MATVIEW:
+			{
+				/*
+				 * Not every table AM uses BLCKSZ wide fixed size
+				 * blocks. Therefore tableam returns the size in bytes - but
+				 * for the purpose of this routine, we want the number of
+				 * blocks. Therefore divide, rounding up.
+				 */
+				uint64 szbytes;
+
+				szbytes = table_relation_size(relation, forkNum);
+
+				return (szbytes + (BLCKSZ - 1)) / BLCKSZ;
+			}
+		case RELKIND_VIEW:
+		case RELKIND_COMPOSITE_TYPE:
+		case RELKIND_FOREIGN_TABLE:
+		case RELKIND_PARTITIONED_TABLE:
+		default:
+			Assert(false);
+			break;
+	}
+
+	return 0; /* satisfy compiler */
 }
 
 /*
diff --git a/src/include/access/tableam.h b/src/include/access/tableam.h
index c018a44267a..00d105af9e5 100644
--- a/src/include/access/tableam.h
+++ b/src/include/access/tableam.h
@@ -533,6 +533,22 @@ typedef struct TableAmRoutine
 										struct ValidateIndexState *state);
 
 
+	/* ------------------------------------------------------------------------
+	 * Miscellaneous functions.
+	 * ------------------------------------------------------------------------
+	 */
+
+	/*
+	 * See table_relation_size().
+	 *
+	 * Note that currently a few use the MAIN_FORKNUM size to vet the validity
+	 * of tids (e.g. nodeTidscans.c), and others use it to figure out the
+	 * range of potentially interesting blocks (brin, analyze). The
+	 * abstraction around this will need to be improved in the near future.
+	 */
+	uint64		(*relation_size) (Relation rel, ForkNumber forkNumber);
+
+
 	/* ------------------------------------------------------------------------
 	 * Planner related functions.
 	 * ------------------------------------------------------------------------
@@ -543,6 +559,10 @@ typedef struct TableAmRoutine
 	 *
 	 * While block oriented, it shouldn't be too hard for an AM that doesn't
 	 * doesn't internally use blocks to convert into a usable representation.
+	 *
+	 * This differs from the relation_size callback by returning size
+	 * estimates (both relation size and tuple count) for planning purposes,
+	 * rather than returning a currently correct estimate.
 	 */
 	void		(*relation_estimate_size) (Relation rel, int32 *attr_widths,
 										   BlockNumber *pages, double *tuples,
@@ -1492,6 +1512,26 @@ table_index_validate_scan(Relation heap_rel,
 }
 
 
+/* ----------------------------------------------------------------------------
+ * Miscellaneous functionality
+ * ----------------------------------------------------------------------------
+ */
+
+/*
+ * Return the current size of `rel` in bytes. If `forkNumber` is
+ * InvalidForkNumber return the relation's overall size, otherwise the size
+ * for the indicated fork.
+ *
+ * Note that the overall size might not be the equivalent of the sum of sizes
+ * for the individual forks for some AMs, e.g. because the AMs storage does
+ * not neatly map onto the builtin types of forks.
+ */
+static inline uint64
+table_relation_size(Relation rel, ForkNumber forkNumber)
+{
+	return rel->rd_tableam->relation_size(rel, forkNumber);
+}
+
 /* ----------------------------------------------------------------------------
  * Planner related functionality
  * ----------------------------------------------------------------------------