diff --git a/src/timezone/localtime.c b/src/timezone/localtime.c
index 154124e..a2faa82 100644
*** a/src/timezone/localtime.c
--- b/src/timezone/localtime.c
*************** static const pg_time_t time_t_min = MINV
*** 55,60 ****
--- 55,67 ----
  static const pg_time_t time_t_max = MAXVAL(pg_time_t, TYPE_BIT(pg_time_t));
  
  /*
+  * We cache the result of trying to load the TZDEFRULES zone here.
+  * tzdefrules_loaded is 0 if not tried yet, +1 if good, -1 if failed.
+  */
+ static struct state tzdefrules_s;
+ static int	tzdefrules_loaded = 0;
+ 
+ /*
   * The DST rules to use if TZ has no rules and we can't load TZDEFRULES.
   * We default to US rules as of 1999-08-17.
   * POSIX 1003.1 section 8.1.1 says that the default DST rules are
*************** tzparse(const char *name, struct state *
*** 942,948 ****
  		charcnt = stdlen + 1;
  		if (sizeof sp->chars < charcnt)
  			return false;
! 		load_ok = tzload(TZDEFRULES, NULL, sp, false) == 0;
  	}
  	if (!load_ok)
  		sp->leapcnt = 0;		/* so, we're off a little */
--- 949,969 ----
  		charcnt = stdlen + 1;
  		if (sizeof sp->chars < charcnt)
  			return false;
! 
! 		/*
! 		 * This bit also differs from the IANA code, which doesn't make any
! 		 * attempt to avoid repetitive loadings of the TZDEFRULES zone.
! 		 */
! 		if (tzdefrules_loaded == 0)
! 		{
! 			if (tzload(TZDEFRULES, NULL, &tzdefrules_s, false) == 0)
! 				tzdefrules_loaded = 1;
! 			else
! 				tzdefrules_loaded = -1;
! 		}
! 		load_ok = (tzdefrules_loaded > 0);
! 		if (load_ok)
! 			memcpy(sp, &tzdefrules_s, sizeof(struct state));
  	}
  	if (!load_ok)
  		sp->leapcnt = 0;		/* so, we're off a little */
diff --git a/src/timezone/pgtz.c b/src/timezone/pgtz.c
index 13ce399..31dbf89 100644
*** a/src/timezone/pgtz.c
--- b/src/timezone/pgtz.c
*************** pg_open_tzfile(const char *name, char *c
*** 80,91 ****
  	int			fullnamelen;
  	int			orignamelen;
  
  	/*
  	 * Loop to split the given name into directory levels; for each level,
  	 * search using scan_directory_ci().
  	 */
- 	strlcpy(fullname, pg_TZDIR(), sizeof(fullname));
- 	orignamelen = fullnamelen = strlen(fullname);
  	fname = name;
  	for (;;)
  	{
--- 80,116 ----
  	int			fullnamelen;
  	int			orignamelen;
  
+ 	/* Initialize fullname with base name of tzdata directory */
+ 	strlcpy(fullname, pg_TZDIR(), sizeof(fullname));
+ 	orignamelen = fullnamelen = strlen(fullname);
+ 
+ 	if (fullnamelen + 1 + strlen(name) >= MAXPGPATH)
+ 		return -1;				/* not gonna fit */
+ 
+ 	/*
+ 	 * If the caller doesn't need the canonical spelling, first just try to
+ 	 * open the name as-is.  This can be expected to succeed if the given name
+ 	 * is already case-correct, or if the filesystem is case-insensitive; and
+ 	 * we don't need to distinguish those situations if we aren't tasked with
+ 	 * reporting the canonical spelling.
+ 	 */
+ 	if (canonname == NULL)
+ 	{
+ 		int			result;
+ 
+ 		fullname[fullnamelen] = '/';
+ 		/* test above ensured this will fit: */
+ 		strcpy(fullname + fullnamelen + 1, name);
+ 		result = open(fullname, O_RDONLY | PG_BINARY, 0);
+ 		if (result >= 0)
+ 			return result;
+ 		/* If that didn't work, fall through to do it the hard way */
+ 	}
+ 
  	/*
  	 * Loop to split the given name into directory levels; for each level,
  	 * search using scan_directory_ci().
  	 */
  	fname = name;
  	for (;;)
  	{
*************** pg_open_tzfile(const char *name, char *c
*** 97,104 ****
  			fnamelen = slashptr - fname;
  		else
  			fnamelen = strlen(fname);
- 		if (fullnamelen + 1 + fnamelen >= MAXPGPATH)
- 			return -1;			/* not gonna fit */
  		if (!scan_directory_ci(fullname, fname, fnamelen,
  							   fullname + fullnamelen + 1,
  							   MAXPGPATH - fullnamelen - 1))
--- 122,127 ----
*************** pg_tzenumerate_next(pg_tzenum *dir)
*** 458,467 ****
  
  		/*
  		 * Load this timezone using tzload() not pg_tzset(), so we don't fill
! 		 * the cache
  		 */
! 		if (tzload(fullname + dir->baselen, dir->tz.TZname, &dir->tz.state,
! 				   true) != 0)
  		{
  			/* Zone could not be loaded, ignore it */
  			continue;
--- 481,491 ----
  
  		/*
  		 * Load this timezone using tzload() not pg_tzset(), so we don't fill
! 		 * the cache.  Also, don't ask for the canonical spelling: we already
! 		 * know it, and pg_open_tzfile's way of finding it out is pretty
! 		 * inefficient.
  		 */
! 		if (tzload(fullname + dir->baselen, NULL, &dir->tz.state, true) != 0)
  		{
  			/* Zone could not be loaded, ignore it */
  			continue;
*************** pg_tzenumerate_next(pg_tzenum *dir)
*** 473,478 ****
--- 497,506 ----
  			continue;
  		}
  
+ 		/* OK, return the canonical zone name spelling. */
+ 		strlcpy(dir->tz.TZname, fullname + dir->baselen,
+ 				sizeof(dir->tz.TZname));
+ 
  		/* Timezone loaded OK. */
  		return &dir->tz;
  	}