diff --git a/src/backend/access/transam/Makefile b/src/backend/access/transam/Makefile
index 3c941a323dc6f3d8d67fe8d9b9b77e9bd04091ac..46252fa71d5b972e4e27713b17b3e91383955da6 100644
--- a/src/backend/access/transam/Makefile
+++ b/src/backend/access/transam/Makefile
@@ -4,7 +4,7 @@
 #    Makefile for access/transam
 #
 # IDENTIFICATION
-#    $Header: /cvsroot/pgsql/src/backend/access/transam/Makefile,v 1.7 1999/09/27 15:47:37 vadim Exp $
+#    $Header: /cvsroot/pgsql/src/backend/access/transam/Makefile,v 1.8 1999/10/24 20:42:27 tgl Exp $
 #
 #-------------------------------------------------------------------------
 
@@ -26,6 +26,10 @@ depend dep:
 clean: 
 	rm -f SUBSYS.o $(OBJS)
 
+# ensure that version checks in xlog.c get recompiled when config.h or
+# catversion.h changes, even if "make depend" hasn't been done.
+xlog.o: xlog.c $(SRCDIR)/include/config.h $(SRCDIR)/include/catalog/catversion.h
+
 ifeq (depend,$(wildcard depend))
 include depend
 endif
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 428ca7998e85a4a23d41ffa52f891220ded06d75..e8e85b111387f552f6a04961697e12a933299aa6 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -1,3 +1,14 @@
+/*------------------------------------------------------------------------- 
+ *
+ * xlog.c
+ *
+ *
+ * Copyright (c) 1994, Regents of the University of California
+ *
+ * $Header: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v 1.6 1999/10/24 20:42:27 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
 #include <fcntl.h>
 #include <unistd.h>
 #include <errno.h>
@@ -5,8 +16,10 @@
 #include <sys/time.h>
 
 #include "postgres.h"
+
 #include "access/xlog.h"
 #include "access/xact.h"
+#include "catalog/catversion.h"
 #include "storage/sinval.h"
 #include "storage/proc.h"
 #include "storage/spin.h"
@@ -99,12 +112,15 @@ typedef struct ControlFileData
 	DBState			state;			/* */
 
 	/*
-	 * following data used to make sure that configurations for this DB
-	 * do not conflict with the backend
+	 * this data is used to make sure that configuration of this DB
+	 * is compatible with the current backend
 	 */
 	uint32			blcksz;			/* block size for this DB */
-	uint32			relseg_size;		/* segmented file's block number */
-	/* MORE DATA FOLLOWS AT THE END OF THIS STRUCTURE
+	uint32			relseg_size;	/* blocks per segment of large relation */
+	uint32			catalog_version_no;	/* internal version number */
+
+	/*
+	 * MORE DATA FOLLOWS AT THE END OF THIS STRUCTURE
 	 * - locations of data dirs 
 	 */
 } ControlFileData;
@@ -1171,6 +1187,7 @@ BootStrapXLOG()
 	ControlFile->state = DB_SHUTDOWNED;
 	ControlFile->blcksz = BLCKSZ;
 	ControlFile->relseg_size = RELSEG_SIZE;
+	ControlFile->catalog_version_no = CATALOG_VERSION_NO;
 
 	if (write(fd, buffer, BLCKSZ) != BLCKSZ)
 		elog(STOP, "BootStrapXLOG failed to write control file: %d", errno);
@@ -1179,9 +1196,6 @@ BootStrapXLOG()
 		elog(STOP, "BootStrapXLOG failed to fsync control file: %d", errno);
 
 	close(fd);
-
-	return;
-
 }
 
 static char*
@@ -1258,11 +1272,16 @@ tryAgain:
 		!XRecOffIsValid(ControlFile->checkPoint.xrecoff))
 		elog(STOP, "Control file context is broken");
 
+	/* Check for incompatible database */
 	if (ControlFile->blcksz != BLCKSZ)
-		elog(STOP, "database was initialized in BLCKSZ(%d), but the backend was compiled in BLCKSZ(%d)",ControlFile->blcksz,BLCKSZ);
-
+		elog(STOP, "database was initialized with BLCKSZ %d,\n\tbut the backend was compiled with BLCKSZ %d.\n\tlooks like you need to initdb.",
+			 ControlFile->blcksz, BLCKSZ);
 	if (ControlFile->relseg_size != RELSEG_SIZE)
-		elog(STOP, "database was initialized in RELSEG_SIZE(%d), but the backend was compiled in RELSEG_SIZE(%d)",ControlFile->relseg_size, RELSEG_SIZE);
+		elog(STOP, "database was initialized with RELSEG_SIZE %d,\n\tbut the backend was compiled with RELSEG_SIZE %d.\n\tlooks like you need to initdb.",
+			 ControlFile->relseg_size, RELSEG_SIZE);
+	if (ControlFile->catalog_version_no != CATALOG_VERSION_NO)
+		elog(STOP, "database was initialized with CATALOG_VERSION_NO %d,\n\tbut the backend was compiled with CATALOG_VERSION_NO %d.\n\tlooks like you need to initdb.",
+			 ControlFile->catalog_version_no, CATALOG_VERSION_NO);
 
 	if (ControlFile->state == DB_SHUTDOWNED)
 		elog(LOG, "Data Base System was shutdowned at %s",
diff --git a/src/include/catalog/catversion.h b/src/include/catalog/catversion.h
new file mode 100644
index 0000000000000000000000000000000000000000..617566290822a08cf9af3e48a3d37637804d2ab9
--- /dev/null
+++ b/src/include/catalog/catversion.h
@@ -0,0 +1,56 @@
+/*------------------------------------------------------------------------- 
+ *
+ * catversion.h
+ *	  "Catalog version number" for Postgres.
+ *
+ * The catalog version number is used to flag incompatible changes in
+ * the Postgres system catalogs.  Whenever anyone changes the format of
+ * a system catalog relation, or adds, deletes, or modifies standard
+ * catalog entries in such a way that an updated backend wouldn't work
+ * with an old database (or vice versa), the catalog version number
+ * should be changed.  The version number stored in pg_control by initdb
+ * is checked against the version number compiled into the backend at
+ * startup time, so that a backend can refuse to run in an incompatible
+ * database.
+ *
+ * The point of this feature is to provide a finer grain of compatibility
+ * checking than is possible from looking at the major version number
+ * stored in PG_VERSION.  It shouldn't matter to end users, but during
+ * development cycles we usually make quite a few incompatible changes
+ * to the contents of the system catalogs, and we don't want to bump the
+ * major version number for each one.  What we can do instead is bump
+ * this internal version number.  This should save some grief for
+ * developers who might otherwise waste time tracking down "bugs" that
+ * are really just code-vs-database incompatibilities.
+ *
+ * The rule for developers is: if you commit a change that requires
+ * an initdb, you should update the catalog version number (as well as
+ * notifying the pghackers mailing list, which has been the informal
+ * practice for a long time).
+ *
+ * The catalog version number is placed here since modifying files in
+ * include/catalog is the most common kind of initdb-forcing change.
+ * But it could be used to protect any kind of incompatible change in
+ * database contents or layout, such as altering tuple headers.
+ *
+ *
+ * Copyright (c) 1994, Regents of the University of California
+ *
+ * $Id: catversion.h,v 1.1 1999/10/24 20:42:26 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef CATVERSION_H
+#define CATVERSION_H
+
+/*
+ * We could use anything we wanted for version numbers, but I recommend
+ * following the "YYYYMMDDN" style often used for DNS zone serial numbers.
+ * YYYYMMDD are the date of the change, and N is the number of the change
+ * on that day.  (Hopefully we'll never commit ten independent sets of
+ * catalog changes on the same day...)
+ */
+
+#define CATALOG_VERSION_NO	199910241
+
+#endif