Fix interaction of log_line_prefix's query_id and log_statement

log_statement is issued before query_id can be computed, so properly
clear the value, and document the interaction.

Reported-by: Fujii Masao, Michael Paquier

Discussion: https://postgr.es/m/YHPkU8hFi4no4NSw@paquier.xyz

Author: Julien Rouhaud

doc/src/sgml/config.sgml

@@ -7139,6 +7139,16 @@ log_line_prefix = '%m [%p] %q%u@%d/%a '
 </programlisting>
         </para>
        </tip>
+
+       <note>
+        <para>
+         The <literal>%Q</literal> escape always reports a zero identifier
+         for lines output by <xref linkend="guc-log-statement"/> because
+         <varname>log_statement</varname> generates output before an
+         identifier can be calculated, including invalid statements for
+         which an identifier cannot be calculated.
+        </para>
+       </note>
       </listitem>
      </varlistentry>
 

src/backend/utils/activity/backend_status.c

@@ -398,6 +398,7 @@ pgstat_bestart(void)
 	lbeentry.st_state = STATE_UNDEFINED;
 	lbeentry.st_progress_command = PROGRESS_COMMAND_INVALID;
 	lbeentry.st_progress_command_target = InvalidOid;
+	lbeentry.st_query_id = UINT64CONST(0);
 
 	/*
 	 * we don't zero st_progress_param here to save cycles; nobody should