Change the pgstat logic so that the stats collector writes the stats file only

upon requests from backends, rather than on a fixed 500msec cycle.  (There's
still throttling logic to ensure it writes no more often than once per
500msec, though.)  This should result in a significant reduction in stats file
write traffic in typical scenarios where the stats are demanded only
infrequently.

This approach also means that the former difficulty with changing
stats_temp_directory on-the-fly has gone away, so remove the caution about
that as well as the thrashing we did to minimize the trouble window.

In passing, also fix pgstat_report_stat() so that we will send a stats
message if we have function call stats but not table stats to report;
this fixes a bug in the recent patch to support function-call stats.

Martin Pihlak

Author: tglsfdc

doc/src/sgml/config.sgml

@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.191 2008/09/30 10:52:09 heikki Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.192 2008/11/03 01:17:08 tgl Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -3347,9 +3347,6 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         improved performance.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
-        If this parameter is changed while the server is running, there is a
-        small window of time until the new statistics file has been written
-        during which the statistics functions might return no information.
        </para>
       </listitem>
      </varlistentry>