Simplify handling of the timezone GUC by making initdb choose the default.

We were doing some amazingly complicated things in order to avoid running
the very expensive identify_system_timezone() procedure during GUC
initialization.  But there is an obvious fix for that, which is to do it
once during initdb and have initdb install the system-specific default into
postgresql.conf, as it already does for most other GUC variables that need
system-environment-dependent defaults.  This means that the timezone (and
log_timezone) settings no longer have any magic behavior in the server.
Per discussion.

doc/src/sgml/config.sgml

@@ -3915,9 +3915,10 @@ FROM pg_stat_activity;
         Sets the time zone used for timestamps written in the server log.
         Unlike <xref linkend="guc-timezone">, this value is cluster-wide,
         so that all sessions will report timestamps consistently.
-        If not explicitly set, the server initializes this variable to the
-        time zone specified by its system environment.  See <xref
-        linkend="datatype-timezones"> for more information.
+        The built-in default is <literal>GMT</>, but that is typically
+        overridden in <filename>postgresql.conf</>; <application>initdb</>
+        will install a setting there corresponding to its system environment.
+        See <xref linkend="datatype-timezones"> for more information.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
        </para>
@@ -4963,9 +4964,10 @@ SET XML OPTION { DOCUMENT | CONTENT };
       <listitem>
        <para>
         Sets the time zone for displaying and interpreting time stamps.
-        If not explicitly set, the server initializes this variable to the
-        time zone specified by its system environment.  See <xref
-        linkend="datatype-timezones"> for more information.
+        The built-in default is <literal>GMT</>, but that is typically
+        overridden in <filename>postgresql.conf</>; <application>initdb</>
+        will install a setting there corresponding to its system environment.
+        See <xref linkend="datatype-timezones"> for more information.
        </para>
       </listitem>
      </varlistentry>

doc/src/sgml/datatype.sgml

@@ -2286,7 +2286,7 @@ January 8 04:05:06 1999 PST
     but continue to be prone to arbitrary changes, particularly with
     respect to daylight-savings rules.
     <productname>PostgreSQL</productname> uses the widely-used
-    <literal>zoneinfo</> time zone database for information about
+    <literal>zoneinfo</> (Olson) time zone database for information about
     historical time zone rules.  For times in the future, the assumption
     is that the latest known rules for a given time zone will
     continue to be observed indefinitely far into the future.
@@ -2432,26 +2432,9 @@ January 8 04:05:06 1999 PST
      The <xref linkend="guc-timezone"> configuration parameter can
      be set in the file <filename>postgresql.conf</>, or in any of the
      other standard ways described in <xref linkend="runtime-config">.
-     There are also several special ways to set it:
+     There are also some special ways to set it:
 
      <itemizedlist>
-      <listitem>
-       <para>
-        If <varname>timezone</> is not specified in
-        <filename>postgresql.conf</> or as a server command-line option,
-        the server attempts to use the value of the <envar>TZ</envar>
-        environment variable as the default time zone.  If <envar>TZ</envar>
-        is not defined or is not any of the time zone names known to
-        <productname>PostgreSQL</productname>, the server attempts to
-        determine the operating system's default time zone by checking the
-        behavior of the C library function <literal>localtime()</>.  The
-        default time zone is selected as the closest match among
-        <productname>PostgreSQL</productname>'s known time zones.
-        (These rules are also used to choose the default value of
-        <xref linkend="guc-log-timezone">, if not specified.)
-       </para>
-      </listitem>
-
       <listitem>
        <para>
         The <acronym>SQL</acronym> command <command>SET TIME ZONE</command>

doc/src/sgml/ref/set.sgml

@@ -239,9 +239,7 @@ SELECT setseed(<replaceable>value</replaceable>);
          <listitem>
           <para>
            Set the time zone to your local time zone (that is, the
-           server's default value of <varname>timezone</>; if this
-           has not been explicitly set anywhere, it will be the zone that
-           the server's operating system defaults to).
+           server's default value of <varname>timezone</>).
           </para>
          </listitem>
         </varlistentry>