diff --git a/src/backend/commands/dbcommands.c b/src/backend/commands/dbcommands.c
index 7df44c9ec41..25dc2f58171 100644
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -13,7 +13,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/dbcommands.c,v 1.227 2009/10/07 22:14:18 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/dbcommands.c,v 1.228 2009/11/12 02:46:16 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -334,20 +334,22 @@ createdb(const CreatedbStmt *stmt)
 	 * Check whether chosen encoding matches chosen locale settings.  This
 	 * restriction is necessary because libc's locale-specific code usually
 	 * fails when presented with data in an encoding it's not expecting. We
-	 * allow mismatch in three cases:
+	 * allow mismatch in four cases:
 	 *
-	 * 1. locale encoding = SQL_ASCII, which means either that the locale is
-	 * C/POSIX which works with any encoding, or that we couldn't determine
+	 * 1. locale encoding = SQL_ASCII, which means that the locale is
+	 * C/POSIX which works with any encoding.
+	 *
+	 * 2. locale encoding = -1, which means that we couldn't determine
 	 * the locale's encoding and have to trust the user to get it right.
 	 *
-	 * 2. selected encoding is SQL_ASCII, but only if you're a superuser. This
-	 * is risky but we have historically allowed it --- notably, the
-	 * regression tests require it.
-	 *
 	 * 3. selected encoding is UTF8 and platform is win32. This is because
 	 * UTF8 is a pseudo codepage that is supported in all locales since it's
 	 * converted to UTF16 before being used.
 	 *
+	 * 4. selected encoding is SQL_ASCII, but only if you're a superuser. This
+	 * is risky but we have historically allowed it --- notably, the
+	 * regression tests require it.
+	 *
 	 * Note: if you change this policy, fix initdb to match.
 	 */
 	ctype_encoding = pg_get_encoding_from_locale(dbctype);
@@ -355,6 +357,7 @@ createdb(const CreatedbStmt *stmt)
 
 	if (!(ctype_encoding == encoding ||
 		  ctype_encoding == PG_SQL_ASCII ||
+		  ctype_encoding == -1 ||
 #ifdef WIN32
 		  encoding == PG_UTF8 ||
 #endif
@@ -369,6 +372,7 @@ createdb(const CreatedbStmt *stmt)
 
 	if (!(collate_encoding == encoding ||
 		  collate_encoding == PG_SQL_ASCII ||
+		  collate_encoding == -1 ||
 #ifdef WIN32
 		  encoding == PG_UTF8 ||
 #endif
diff --git a/src/backend/utils/mb/mbutils.c b/src/backend/utils/mb/mbutils.c
index 5aaaae502f8..c8a43dced6f 100644
--- a/src/backend/utils/mb/mbutils.c
+++ b/src/backend/utils/mb/mbutils.c
@@ -4,7 +4,7 @@
  *
  * Tatsuo Ishii
  *
- * $PostgreSQL: pgsql/src/backend/utils/mb/mbutils.c,v 1.91 2009/10/17 05:14:52 mha Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/mb/mbutils.c,v 1.92 2009/11/12 02:46:16 tgl Exp $
  */
 #include "postgres.h"
 
@@ -984,7 +984,14 @@ int
 GetPlatformEncoding(void)
 {
 	if (PlatformEncoding == NULL)
-		PlatformEncoding = &pg_enc2name_tbl[pg_get_encoding_from_locale("")];
+	{
+		/* try to determine encoding of server's environment locale */
+		int		encoding = pg_get_encoding_from_locale("");
+
+		if (encoding < 0)
+			encoding = PG_SQL_ASCII;
+		PlatformEncoding = &pg_enc2name_tbl[encoding];
+	}
 	return PlatformEncoding->encoding;
 }
 
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index ae400004a22..4a052b0167c 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -42,7 +42,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  * Portions taken from FreeBSD.
  *
- * $PostgreSQL: pgsql/src/bin/initdb/initdb.c,v 1.175 2009/09/03 01:40:11 tgl Exp $
+ * $PostgreSQL: pgsql/src/bin/initdb/initdb.c,v 1.176 2009/11/12 02:46:16 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2193,21 +2193,14 @@ check_locale_encoding(const char *locale, int user_enc)
 
 	locale_enc = pg_get_encoding_from_locale(locale);
 
-	/* We allow selection of SQL_ASCII --- see notes in createdb() */
+	/* See notes in createdb() to understand these tests */
 	if (!(locale_enc == user_enc ||
 		  locale_enc == PG_SQL_ASCII ||
-		  user_enc == PG_SQL_ASCII
+		  locale_enc == -1 ||
 #ifdef WIN32
-
-	/*
-	 * On win32, if the encoding chosen is UTF8, all locales are OK (assuming
-	 * the actual locale name passed the checks above). This is because UTF8
-	 * is a pseudo-codepage, that we convert to UTF16 before doing any
-	 * operations on, and UTF16 supports all locales.
-	 */
-		  || user_enc == PG_UTF8
+		  user_enc == PG_UTF8 ||
 #endif
-		  ))
+		  user_enc == PG_SQL_ASCII))
 	{
 		fprintf(stderr, _("%s: encoding mismatch\n"), progname);
 		fprintf(stderr,
@@ -2851,11 +2844,9 @@ main(int argc, char *argv[])
 
 		ctype_enc = pg_get_encoding_from_locale(lc_ctype);
 
-		if (ctype_enc == PG_SQL_ASCII &&
-			!(pg_strcasecmp(lc_ctype, "C") == 0 ||
-			  pg_strcasecmp(lc_ctype, "POSIX") == 0))
+		if (ctype_enc == -1)
 		{
-			/* Hmm, couldn't recognize the locale's codeset */
+			/* Couldn't recognize the locale's codeset */
 			fprintf(stderr, _("%s: could not find suitable encoding for locale %s\n"),
 					progname, lc_ctype);
 			fprintf(stderr, _("Rerun %s with the -E option.\n"), progname);
diff --git a/src/port/chklocale.c b/src/port/chklocale.c
index 864071d05e6..dd10887a34e 100644
--- a/src/port/chklocale.c
+++ b/src/port/chklocale.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/port/chklocale.c,v 1.11 2009/02/10 19:29:39 petere Exp $
+ *	  $PostgreSQL: pgsql/src/port/chklocale.c,v 1.12 2009/11/12 02:46:16 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -181,6 +181,8 @@ static const struct encoding_match encoding_match_list[] = {
 
 	{PG_SHIFT_JIS_2004, "SJIS_2004"},
 
+	{PG_SQL_ASCII, "US-ASCII"},
+
 	{PG_SQL_ASCII, NULL}		/* end marker */
 };
 
@@ -215,13 +217,13 @@ win32_langinfo(const char *ctype)
 
 /*
  * Given a setting for LC_CTYPE, return the Postgres ID of the associated
- * encoding, if we can determine it.
+ * encoding, if we can determine it.  Return -1 if we can't determine it.
  *
  * Pass in NULL to get the encoding for the current locale setting.
+ * Pass "" to get the encoding selected by the server's environment.
  *
  * If the result is PG_SQL_ASCII, callers should treat it as being compatible
- * with any desired encoding.  We return this if the locale is C/POSIX or we
- * can't determine the encoding.
+ * with any desired encoding.
  */
 int
 pg_get_encoding_from_locale(const char *ctype)
@@ -237,17 +239,17 @@ pg_get_encoding_from_locale(const char *ctype)
 
 		save = setlocale(LC_CTYPE, NULL);
 		if (!save)
-			return PG_SQL_ASCII;	/* setlocale() broken? */
+			return -1;				/* setlocale() broken? */
 		/* must copy result, or it might change after setlocale */
 		save = strdup(save);
 		if (!save)
-			return PG_SQL_ASCII;	/* out of memory; unlikely */
+			return -1;				/* out of memory; unlikely */
 
 		name = setlocale(LC_CTYPE, ctype);
 		if (!name)
 		{
 			free(save);
-			return PG_SQL_ASCII;	/* bogus ctype passed in? */
+			return -1;				/* bogus ctype passed in? */
 		}
 
 #ifndef WIN32
@@ -266,7 +268,7 @@ pg_get_encoding_from_locale(const char *ctype)
 		/* much easier... */
 		ctype = setlocale(LC_CTYPE, NULL);
 		if (!ctype)
-			return PG_SQL_ASCII;	/* setlocale() broken? */
+			return -1;				/* setlocale() broken? */
 #ifndef WIN32
 		sys = nl_langinfo(CODESET);
 		if (sys)
@@ -277,7 +279,7 @@ pg_get_encoding_from_locale(const char *ctype)
 	}
 
 	if (!sys)
-		return PG_SQL_ASCII;	/* out of memory; unlikely */
+		return -1;					/* out of memory; unlikely */
 
 	/* If locale is C or POSIX, we can allow all encodings */
 	if (pg_strcasecmp(ctype, "C") == 0 || pg_strcasecmp(ctype, "POSIX") == 0)
@@ -328,12 +330,16 @@ pg_get_encoding_from_locale(const char *ctype)
 #endif
 
 	free(sys);
-	return PG_SQL_ASCII;
+	return -1;
 }
 #else							/* (HAVE_LANGINFO_H && CODESET) || WIN32 */
 
 /*
  * stub if no platform support
+ *
+ * Note: we could return -1 here, but that would have the effect of
+ * forcing users to specify an encoding to initdb on such platforms.
+ * It seems better to silently default to SQL_ASCII.
  */
 int
 pg_get_encoding_from_locale(const char *ctype)