pgstat-mydatabase-fix-prototype.diff

diff --git i/src/backend/utils/activity/pgstat_database.c w/src/backend/utils/activity/pgstat_database.c
index 7149f22f729..3efcc9bb7c3 100644
--- i/src/backend/utils/activity/pgstat_database.c
+++ w/src/backend/utils/activity/pgstat_database.c
@@ -271,6 +271,14 @@ pgstat_update_dbstats(TimestampTz ts)
 {
 	PgStat_StatDBEntry *dbentry;
 
+	/*
+	 * If we haven't connected to a database yet, don't attribute time to
+	 * "shared state" (InvalidOid is used to track stats for shared relations
+	 * etc).
+	 */
+	if (!OidIsValid(MyDatabaseId))
+		return;
+
 	dbentry = pgstat_prep_database_pending(MyDatabaseId);
 
 	/*
@@ -327,6 +335,12 @@ pgstat_prep_database_pending(Oid dboid)
 {
 	PgStat_EntryRef *entry_ref;
 
+	/*
+	 * Shouldn't report stats on database objects before having connected to
+	 * database.
+	 */
+	Assert(!OidIsValid(dboid) || OidIsValid(MyDatabaseId));
+
 	entry_ref = pgstat_prep_pending_entry(PGSTAT_KIND_DATABASE, dboid, InvalidOid,
 										  NULL);
 
diff --git i/src/backend/utils/init/postinit.c w/src/backend/utils/init/postinit.c
index 561bd13ed24..a40f70503a4 100644
--- i/src/backend/utils/init/postinit.c
+++ w/src/backend/utils/init/postinit.c
@@ -1006,7 +1006,7 @@ InitPostgres(const char *in_dbname, Oid dboid,
 	 */
 	if (bootstrap)
 	{
-		MyDatabaseId = Template1DbOid;
+		dboid = Template1DbOid;
 		MyDatabaseTableSpace = DEFAULTTABLESPACE_OID;
 	}
 	else if (in_dbname != NULL)
@@ -1020,32 +1020,9 @@ InitPostgres(const char *in_dbname, Oid dboid,
 					(errcode(ERRCODE_UNDEFINED_DATABASE),
 					 errmsg("database \"%s\" does not exist", in_dbname)));
 		dbform = (Form_pg_database) GETSTRUCT(tuple);
-		MyDatabaseId = dbform->oid;
-		MyDatabaseTableSpace = dbform->dattablespace;
-		/* take database name from the caller, just for paranoia */
-		strlcpy(dbname, in_dbname, sizeof(dbname));
+		dboid = dbform->oid;
 	}
-	else if (OidIsValid(dboid))
-	{
-		/* caller specified database by OID */
-		HeapTuple	tuple;
-		Form_pg_database dbform;
-
-		tuple = GetDatabaseTupleByOid(dboid);
-		if (!HeapTupleIsValid(tuple))
-			ereport(FATAL,
-					(errcode(ERRCODE_UNDEFINED_DATABASE),
-					 errmsg("database %u does not exist", dboid)));
-		dbform = (Form_pg_database) GETSTRUCT(tuple);
-		MyDatabaseId = dbform->oid;
-		MyDatabaseTableSpace = dbform->dattablespace;
-		Assert(MyDatabaseId == dboid);
-		strlcpy(dbname, NameStr(dbform->datname), sizeof(dbname));
-		/* pass the database name back to the caller */
-		if (out_dbname)
-			strcpy(out_dbname, dbname);
-	}
-	else
+	else if (!OidIsValid(dboid))
 	{
 		/*
 		 * If this is a background worker not bound to any particular
@@ -1083,8 +1060,47 @@ InitPostgres(const char *in_dbname, Oid dboid,
 	 * CREATE DATABASE.
 	 */
 	if (!bootstrap)
-		LockSharedObject(DatabaseRelationId, MyDatabaseId, 0,
-						 RowExclusiveLock);
+		LockSharedObject(DatabaseRelationId, dboid, 0, RowExclusiveLock);
+
+	/*
+	 * Recheck pg_database to make sure the target database hasn't gone away.
+	 * If there was a concurrent DROP DATABASE, this ensures we will die
+	 * cleanly without creating a mess.
+	 */
+	if (!bootstrap)
+	{
+		HeapTuple	tuple;
+		Form_pg_database dbform;
+
+		tuple = GetDatabaseTupleByOid(dboid);
+		if (HeapTupleIsValid(tuple))
+			dbform = (Form_pg_database) GETSTRUCT(tuple);
+
+		if (!HeapTupleIsValid(tuple) ||
+			(in_dbname && namestrcmp(&dbform->datname, in_dbname)))
+		{
+			if (in_dbname)
+				ereport(PANIC,
+						(errcode(ERRCODE_UNDEFINED_DATABASE),
+						 errmsg("database \"%s\" does not exist", in_dbname),
+						 errdetail("It seems to have just been dropped or renamed.")));
+			else
+				ereport(FATAL,
+						(errcode(ERRCODE_UNDEFINED_DATABASE),
+						 errmsg("database %u does not exist", dboid)));
+		}
+
+		MyDatabaseTableSpace = dbform->dattablespace;
+		strlcpy(dbname, NameStr(dbform->datname), sizeof(dbname));
+		if (out_dbname)
+			strcpy(out_dbname, dbname);
+	}
+
+	/*
+	 * Now that we rechecked, we can be certain we're connected to a database
+	 * and thus can set MyDatabaseId.
+	 */
+	MyDatabaseId = dboid;
 
 	/*
 	 * Now we can mark our PGPROC entry with the database ID.
@@ -1108,25 +1124,6 @@ InitPostgres(const char *in_dbname, Oid dboid,
 	 */
 	InvalidateCatalogSnapshot();
 
-	/*
-	 * Recheck pg_database to make sure the target database hasn't gone away.
-	 * If there was a concurrent DROP DATABASE, this ensures we will die
-	 * cleanly without creating a mess.
-	 */
-	if (!bootstrap)
-	{
-		HeapTuple	tuple;
-
-		tuple = GetDatabaseTuple(dbname);
-		if (!HeapTupleIsValid(tuple) ||
-			MyDatabaseId != ((Form_pg_database) GETSTRUCT(tuple))->oid ||
-			MyDatabaseTableSpace != ((Form_pg_database) GETSTRUCT(tuple))->dattablespace)
-			ereport(FATAL,
-					(errcode(ERRCODE_UNDEFINED_DATABASE),
-					 errmsg("database \"%s\" does not exist", dbname),
-					 errdetail("It seems to have just been dropped or renamed.")));
-	}
-
 	/*
 	 * Now we should be able to access the database directory safely. Verify
 	 * it's there and looks reasonable.