diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 1f6fba5f75..3ac3254afb 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -324,7 +324,7 @@ CheckMyDatabase(const char *name, bool am_superuser)
 					PGC_INTERNAL, PGC_S_OVERRIDE);
 	/* If we have no other source of client_encoding, use server encoding */
 	SetConfigOption("client_encoding", GetDatabaseEncodingName(),
-					PGC_BACKEND, PGC_S_DEFAULT);
+					PGC_BACKEND, PGC_S_DYNAMIC_DEFAULT);
 
 	/* assign locale variables */
 	collate = NameStr(dbform->datcollate);
diff --git a/src/backend/utils/misc/guc-file.l b/src/backend/utils/misc/guc-file.l
index 10ef12eb24..78907b939d 100644
--- a/src/backend/utils/misc/guc-file.l
+++ b/src/backend/utils/misc/guc-file.l
@@ -14,6 +14,7 @@
 #include <ctype.h>
 #include <unistd.h>
 
+#include "mb/pg_wchar.h"
 #include "miscadmin.h"
 #include "storage/fd.h"
 #include "utils/guc.h"
@@ -109,7 +110,6 @@ ProcessConfigFile(GucContext context)
 				   *tail;
 	char	   *cvc = NULL;
 	struct config_string *cvc_struct;
-	const char *envvar;
 	int			i;
 
 	Assert(context == PGC_POSTMASTER || context == PGC_SIGHUP);
@@ -265,7 +265,7 @@ ProcessConfigFile(GucContext context)
 				stack->source = PGC_S_DEFAULT;
 		}
 
-		/* Now we can re-apply the wired-in default */
+		/* Now we can re-apply the wired-in default (i.e., the boot_val) */
 		set_config_option(gconf->name, NULL, context, PGC_S_DEFAULT,
 						  GUC_ACTION_SET, true);
 		if (context == PGC_SIGHUP)
@@ -275,25 +275,28 @@ ProcessConfigFile(GucContext context)
 	}
 
 	/*
-	 * Restore any variables determined by environment variables.  This
-	 * is a no-op except in the case where one of these had been in the
-	 * config file and is now removed.  PGC_S_ENV_VAR will override the
-	 * wired-in default we just applied, but cannot override any other source.
+	 * Restore any variables determined by environment variables or
+	 * dynamically-computed defaults.  This is a no-op except in the case
+	 * where one of these had been in the config file and is now removed.
 	 *
-	 * Keep this list in sync with InitializeGUCOptions()!
-	 * PGPORT can be ignored, because it cannot be changed without restart.
-	 * We assume rlimit hasn't changed, either.
+	 * In particular, we *must not* do this during the postmaster's
+	 * initial loading of the file, since the timezone functions in
+	 * particular should be run only after initialization is complete.
+	 *
+	 * XXX this is an unmaintainable crock, because we have to know how
+	 * to set (or at least what to call to set) every variable that could
+	 * potentially have PGC_S_DYNAMIC_DEFAULT or PGC_S_ENV_VAR source.
+	 * However, there's no time to redesign it for 9.1.
 	 */
-	envvar = getenv("PGDATESTYLE");
-	if (envvar != NULL)
-		set_config_option("datestyle", envvar, PGC_POSTMASTER,
-						  PGC_S_ENV_VAR, GUC_ACTION_SET, true);
-
-	envvar = getenv("PGCLIENTENCODING");
-	if (envvar != NULL)
-		set_config_option("client_encoding", envvar, PGC_POSTMASTER,
-						  PGC_S_ENV_VAR, GUC_ACTION_SET, true);
-
+	if (context == PGC_SIGHUP)
+	{
+		InitializeGUCOptionsFromEnvironment();
+		pg_timezone_initialize();
+		pg_timezone_abbrev_initialize();
+		/* this selects SQL_ASCII in processes not connected to a database */
+		SetConfigOption("client_encoding", GetDatabaseEncodingName(),
+						PGC_BACKEND, PGC_S_DYNAMIC_DEFAULT);
+	}
 
 	/* If we got here all the options checked out okay, so apply them. */
 	for (item = head; item; item = item->next)
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 738e2152ba..92391eda2f 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -502,6 +502,7 @@ const char *const GucContext_Names[] =
 const char *const GucSource_Names[] =
 {
 	 /* PGC_S_DEFAULT */ "default",
+	 /* PGC_S_DYNAMIC_DEFAULT */ "default",
 	 /* PGC_S_ENV_VAR */ "environment variable",
 	 /* PGC_S_FILE */ "configuration file",
 	 /* PGC_S_ARGV */ "command line",
@@ -3269,6 +3270,7 @@ static int	GUCNestLevel = 0;	/* 1 when in main transaction */
 
 static int	guc_var_compare(const void *a, const void *b);
 static int	guc_name_compare(const char *namea, const char *nameb);
+static void InitializeGUCOptionsFromEnvironment(void);
 static void InitializeOneGUCOption(struct config_generic * gconf);
 static void push_old_value(struct config_generic * gconf, GucAction action);
 static void ReportGUCOption(struct config_generic * record);
@@ -3812,8 +3814,6 @@ void
 InitializeGUCOptions(void)
 {
 	int			i;
-	char	   *env;
-	long		stack_rlimit;
 
 	/*
 	 * Before log_line_prefix could possibly receive a nonempty setting, make
@@ -3852,9 +3852,25 @@ InitializeGUCOptions(void)
 
 	/*
 	 * For historical reasons, some GUC parameters can receive defaults from
-	 * environment variables.  Process those settings.	NB: if you add or
-	 * remove anything here, see also ProcessConfigFile().
+	 * environment variables.  Process those settings.
 	 */
+	InitializeGUCOptionsFromEnvironment();
+}
+
+/*
+ * Assign any GUC values that can come from the server's environment.
+ *
+ * This is called from InitializeGUCOptions, and also from ProcessConfigFile
+ * to deal with the possibility that a setting has been removed from
+ * postgresql.conf and should now get a value from the environment.
+ * (The latter is a kludge that should probably go away someday; if so,
+ * fold this back into InitializeGUCOptions.)
+ */
+static void
+InitializeGUCOptionsFromEnvironment(void)
+{
+	char	   *env;
+	long		stack_rlimit;
 
 	env = getenv("PGPORT");
 	if (env != NULL)
@@ -6334,6 +6350,7 @@ define_custom_variable(struct config_generic * variable)
 	switch (pHolder->gen.source)
 	{
 		case PGC_S_DEFAULT:
+		case PGC_S_DYNAMIC_DEFAULT:
 		case PGC_S_ENV_VAR:
 		case PGC_S_FILE:
 		case PGC_S_ARGV:
@@ -8420,15 +8437,13 @@ assign_timezone_abbreviations(const char *newval, void *extra)
  *
  * This is called after initial loading of postgresql.conf.  If no
  * timezone_abbreviations setting was found therein, select default.
+ * If a non-default value is already installed, nothing will happen.
  */
 void
 pg_timezone_abbrev_initialize(void)
 {
-	if (timezone_abbreviations_string == NULL)
-	{
-		SetConfigOption("timezone_abbreviations", "Default",
-						PGC_POSTMASTER, PGC_S_DEFAULT);
-	}
+	SetConfigOption("timezone_abbreviations", "Default",
+					PGC_POSTMASTER, PGC_S_DYNAMIC_DEFAULT);
 }
 
 static const char *
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index 5a42d8cec3..ee52cd735e 100644
--- a/src/include/utils/guc.h
+++ b/src/include/utils/guc.h
@@ -82,7 +82,8 @@ typedef enum
  */
 typedef enum
 {
-	PGC_S_DEFAULT,				/* wired-in default */
+	PGC_S_DEFAULT,				/* hard-wired default ("boot_val") */
+	PGC_S_DYNAMIC_DEFAULT,		/* default computed during initialization */
 	PGC_S_ENV_VAR,				/* postmaster environment variable */
 	PGC_S_FILE,					/* postgresql.conf */
 	PGC_S_ARGV,					/* postmaster command line */
diff --git a/src/timezone/pgtz.c b/src/timezone/pgtz.c
index cb66f6380b..622cf94c5a 100644
--- a/src/timezone/pgtz.c
+++ b/src/timezone/pgtz.c
@@ -1438,6 +1438,10 @@ pg_timezone_pre_initialize(void)
  * This is called after initial loading of postgresql.conf.  If no TimeZone
  * setting was found therein, we try to derive one from the environment.
  * Likewise for log_timezone.
+ *
+ * Note: this is also called from ProcessConfigFile, to re-establish valid
+ * GUC settings if the GUCs have been reset to default following their
+ * removal from postgresql.conf.
  */
 void
 pg_timezone_initialize(void)
@@ -1463,21 +1467,34 @@ pg_timezone_initialize(void)
 		log_timezone = def_tz;
 	}
 
-	/* Now, set the timezone GUC if it's not already set */
-	if (GetConfigOption("timezone", false) == NULL)
-	{
-		/* Tell GUC about the value. Will redundantly call pg_tzset() */
+	/*
+	 * Now, set the timezone and log_timezone GUCs if they're still default.
+	 * (This will redundantly call pg_tzset().)
+	 *
+	 * We choose to label these values PGC_S_ENV_VAR, rather than
+	 * PGC_S_DYNAMIC_DEFAULT which would be functionally equivalent, because
+	 * they came either from getenv("TZ") or from libc behavior that's
+	 * determined by process environment of some kind.
+	 *
+	 * Note: in the case where a setting has just been removed from
+	 * postgresql.conf, this code will not do what you might expect, namely
+	 * call select_default_timezone() and install that value as the setting.
+	 * Rather, the previously active setting --- typically the one from
+	 * postgresql.conf --- will be reinstalled, relabeled as PGC_S_ENV_VAR.
+	 * If we did try to install the "correct" default value, the effect would
+	 * be that each postmaster child would independently run an extremely
+	 * expensive search of the timezone database, bringing the database to its
+	 * knees for possibly multiple seconds.  This is so unpleasant, and could
+	 * so easily be triggered quite unintentionally, that it seems better to
+	 * violate the principle of least astonishment.
+	 */
+	if (GetConfigOptionResetString("timezone") == NULL)
 		SetConfigOption("timezone", pg_get_timezone_name(session_timezone),
 						PGC_POSTMASTER, PGC_S_ENV_VAR);
-	}
 
-	/* Likewise for log timezone */
-	if (GetConfigOption("log_timezone", false) == NULL)
-	{
-		/* Tell GUC about the value. Will redundantly call pg_tzset() */
+	if (GetConfigOptionResetString("log_timezone") == NULL)
 		SetConfigOption("log_timezone", pg_get_timezone_name(log_timezone),
 						PGC_POSTMASTER, PGC_S_ENV_VAR);
-	}
 }