Access to the table
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
= operator to compare the constant in the query to
the stored value. Thus the data in
pg_statistic 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
pg_statistic to a user who should not be able
to see it.
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
SELECT privilege on the table or
the involved columns, or the operator used must be
LEAKPROOF (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.
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.
This restriction applies only to cases where the planner would
need to execute a user-defined operator on one or more values from
pg_statistic. 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.
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.
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