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>58.2. Planner Statistics and Security</A
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><P
> Access to the table <TT
CLASS="STRUCTNAME"
>pg_statistic</TT
> 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 <TT
CLASS="LITERAL"
>=</TT
>
operator to compare the constant in the query to the stored value.
Thus the data in <TT
CLASS="STRUCTNAME"
>pg_statistic</TT
> 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
<TT
CLASS="STRUCTNAME"
>pg_statistic</TT
> to a user who should not be able to
see it.
</P
><P
> 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 <TT
CLASS="LITERAL"
>SELECT</TT
> privilege on the table or the involved
columns, or the operator used must be <TT
CLASS="LITERAL"
>LEAKPROOF</TT
> (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.
</P
><P
> 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 leak-proof
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.
</P
><P
> This restriction applies only to cases where the planner would need to
execute a user-defined operator on one or more values
from <TT
CLASS="STRUCTNAME"
>pg_statistic</TT
>. 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.
</P
><P
> 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.
</P
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