more response
    messages depending on the contents of the query command string,
    and finally a ReadyForQuery response message.  ReadyForQuery
    informs the frontend that it can safely send a new command.
    (It is not actually necessary for the frontend to wait for
    ReadyForQuery before issuing another command, but the frontend must
    then take responsibility for figuring out what happens if the earlier
    command fails and already-issued later commands succeed.)
   </para>

   <para>
    The possible response messages from the backend are:

    <variablelist>
     <varlistentry>
      <term>CommandComplete</term>
      <listitem>
       <para>
        An SQL command completed normally.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>CopyInResponse</term>
      <listitem>
       <para>
        The backend is ready to copy data from the frontend to a
        table; see <xref linkend="protocol-copy"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>CopyOutResponse</term>
      <listitem>
       <para>
        The backend is ready to copy data from a table to the
        frontend; see <xref linkend="protocol-copy"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>RowDescription</term>
      <listitem>
       <para>
        Indicates that rows are about to be returned in response to
        a <command>SELECT</command>, <command>FETCH</command>, etc. query.
        The contents of this message describe the column layout of the rows.
        This will be followed by a DataRow message for each row being returned
        to the frontend.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>DataRow</term>
      <listitem>
       <para>
        One of the set of rows returned by
        a <command>SELECT</command>, <command>FETCH</command>, etc. query.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>EmptyQueryResponse</term>
      <listitem>
       <para>
        An empty query string was recognized.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ErrorResponse</term>
      <listitem>
       <para>
        An error has occurred.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ReadyForQuery</term>
      <listitem>
       <para>
        Processing of the query string is complete.  A separate
        message is sent to indicate this because the query string might
        contain multiple SQL commands.  (CommandComplete marks the
        end of processing one SQL command, not the whole string.)
        ReadyForQuery will always be sent, whether processing
        terminates successfully or with an error.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>NoticeResponse</term>
      <listitem>
       <para>
        A warning message has been issued in relation to the query.
        Notices are in addition to other responses, i.e., the backend
        will continue processing the command.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>

   <para>
    The response to a <command>SELECT</command> query (or other queries that
    return row sets, such as <command>EXPLAIN</command> or <command>SHOW</command>)
    normally consists of RowDescription, zero or more
    DataRow messages, and then CommandComplete.
    <command>COPY</command> to or from the frontend invokes special protocol
    as described in <xref linkend="protocol-copy"/>.
    All other query types normally produce only
    a CommandComplete message.
   </para>

   <para>
    Since a query string could contain several queries (separated by
    semicolons), there might be several such response sequences before the
    backend finishes processing the query string.  ReadyForQuery is issued
    when the entire string has been processed