remember_extension_size.diff

diff --git i/src/include/access/hio.h w/src/include/access/hio.h
index 228433ee4a2..5ae39aec7f8 100644
--- i/src/include/access/hio.h
+++ w/src/include/access/hio.h
@@ -41,6 +41,7 @@ typedef struct BulkInsertStateData
 	 */
 	BlockNumber next_free;
 	BlockNumber last_free;
+	uint32		already_extended_by;
 } BulkInsertStateData;
 
 
diff --git i/src/backend/access/heap/heapam.c w/src/backend/access/heap/heapam.c
index 7ed72abe597..6a66214a580 100644
--- i/src/backend/access/heap/heapam.c
+++ w/src/backend/access/heap/heapam.c
@@ -1776,6 +1776,7 @@ GetBulkInsertState(void)
 	bistate->current_buf = InvalidBuffer;
 	bistate->next_free = InvalidBlockNumber;
 	bistate->last_free = InvalidBlockNumber;
+	bistate->already_extended_by = 0;
 	return bistate;
 }
 
diff --git i/src/backend/access/heap/hio.c w/src/backend/access/heap/hio.c
index c275b08494d..a2be4273df1 100644
--- i/src/backend/access/heap/hio.c
+++ w/src/backend/access/heap/hio.c
@@ -283,6 +283,13 @@ RelationAddBlocks(Relation relation, BulkInsertState bistate,
 		 */
 		extend_by_pages += extend_by_pages * waitcount;
 
+		/*
+		 * If we previously extended using the same bistate, it's very likely
+		 * we'll extend some more. Try to extend by as many pages.
+		 */
+		if (bistate)
+			extend_by_pages = Max(extend_by_pages, bistate->already_extended_by);
+
 		/*
 		 * Can't extend by more than MAX_BUFFERS_TO_EXTEND_BY, we need to pin
 		 * them all concurrently.
@@ -409,6 +416,7 @@ RelationAddBlocks(Relation relation, BulkInsertState bistate,
 		/* maintain bistate->current_buf */
 		IncrBufferRefCount(buffer);
 		bistate->current_buf = buffer;
+		bistate->already_extended_by += extend_by_pages;
 	}
 
 	return buffer;