OFF) SELECT * FROM t WHERE a <= 49 AND b > 49;
                                QUERY PLAN
-------------------------------------------------------------------&zwsp;--------
 Seq Scan on t  (cost=0.00..195.00 rows=1 width=8) (actual rows=0.00 loops=1)
   Filter: ((a <= 49) AND (b > 49))
   Rows Removed by Filter: 10000
</programlisting>

   </para>

  </sect2>

 </sect1>

 <sect1 id="planner-stats-security">
  <title>Planner Statistics and Security</title>

  <para>
   Access to the table <structname>pg_statistic</structname> is restricted to
   superusers, so that ordinary users cannot learn about the contents of the
   tables of other users from it.  Some selectivity estimation functions will
   use a user-provided operator (either the operator appearing in the query or
   a related operator) to analyze the stored statistics.  For example, in order
   to determine whether a stored most common value is applicable, the
   selectivity estimator will have to run the appropriate <literal>=</literal>
   operator to compare the constant in the query to the stored value.
   Thus the data in <structname>pg_statistic</structname> is potentially
   passed to user-defined operators.  An appropriately crafted operator can
   intentionally leak the passed operands (for example, by logging them
   or writing them to a different table), or accidentally leak them by showing
   their values in error messages, in either case possibly exposing data from
   <structname>pg_statistic</structname> to a user who should not be able to
   see it.
  </para>

  <para>
   In order to prevent this, the following applies to all built-in selectivity
   estimation functions.  When planning a query, in order to be able to use
   stored statistics, the current user must either
   have <literal>SELECT</literal> privilege on the table or the involved
   columns, or the operator used must be <literal>LEAKPROOF</literal> (more
   accurately, the function that the operator is based on).  If not, then the
   selectivity estimator will behave as if no statistics are available, and
   the planner will proceed with default or fall-back assumptions.
   The <xref linkend="app-psql"/> program's
   <command><link linkend="app-psql-meta-command-do-lc">\do+</link></command>
   meta-command is useful to determine which operators are marked as leakproof.
  </para>

  <para>
   If a user does not have the required privilege on the table or columns,
   then in many cases the query will ultimately receive a permission-denied
   error, in which case this mechanism is invisible in practice.  But if the
   user is reading from a security-barrier view, then the planner might wish
   to check the statistics of an underlying table that is otherwise
   inaccessible to the user.  In that case, the operator should be leakproof
   or the statistics will not be used.  There is no direct feedback about
   that, except that the plan might be suboptimal.  If one suspects that this
   is the case, one could try running the query as a more privileged user,
   to see if a different plan results.
  </para>

  <para>
   This restriction applies only to cases where the planner would need to
   execute a user-defined operator on one or more values
   from <structname>pg_statistic</structname>.  Thus the planner is permitted
   to use generic statistical information, such as the fraction of null values
   or the number of distinct values in a column, regardless of access
   privileges.
  </para>

  <para>
   Selectivity estimation functions contained in third-party extensions that
   potentially operate on statistics with user-defined operators should follow
   the same security rules.  Consult the PostgreSQL source code for guidance.
  </para>
 </sect1>
</chapter>