When using the WAL-logged CREATE DATABASE strategy, bulk extend.

This should improve performance, and was suggested by Andres Freund.
Back-patch to v15 to keep the code consistent across branches.

Dilip Kumar

Discussion: http://postgr.es/m/C3458199-FEDD-4356-865A-08DFAA5D4065@anarazel.de
Discussion: http://postgr.es/m/CAFiTN-sJ0vVpJrZ=R5M+g7Tr8=NN4wKOtrqOcDEsfFfnZgivVA@mail.gmail.com

src/backend/storage/buffer/bufmgr.c

@@ -3712,6 +3712,7 @@ RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
 	bool		use_wal;
 	BlockNumber nblocks;
 	BlockNumber blkno;
+	PGAlignedBlock buf;
 	BufferAccessStrategy bstrategy_src;
 	BufferAccessStrategy bstrategy_dst;
 
@@ -3730,6 +3731,14 @@ RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
 	if (nblocks == 0)
 		return;
 
+	/*
+	 * Bulk extend the destination relation of the same size as the source
+	 * relation before starting to copy block by block.
+	 */
+	memset(buf.data, 0, BLCKSZ);
+	smgrextend(smgropen(dstlocator, InvalidBackendId), forkNum, nblocks - 1,
+			   buf.data, true);
+
 	/* This is a bulk operation, so use buffer access strategies. */
 	bstrategy_src = GetAccessStrategy(BAS_BULKREAD);
 	bstrategy_dst = GetAccessStrategy(BAS_BULKWRITE);
@@ -3747,7 +3756,7 @@ RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
 		srcPage = BufferGetPage(srcBuf);
 
 		/* Use P_NEW to extend the destination relation. */
-		dstBuf = ReadBufferWithoutRelcache(dstlocator, forkNum, P_NEW,
+		dstBuf = ReadBufferWithoutRelcache(dstlocator, forkNum, blkno,
 										   RBM_NORMAL, bstrategy_dst,
 										   permanent);
 		LockBuffer(dstBuf, BUFFER_LOCK_EXCLUSIVE);