<!--
doc/src/sgml/ref/create_policy.sgml
PostgreSQL documentation
-->

<refentry id="sql-createpolicy">
 <indexterm zone="sql-createpolicy">
  <primary>CREATE POLICY</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>CREATE POLICY</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE POLICY</refname>
  <refpurpose>define a new row-level security policy for a table</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable class="parameter">table_name</replaceable>
    [ AS { PERMISSIVE | RESTRICTIVE } ]
    [ FOR { ALL | SELECT | INSERT | UPDATE | DELETE } ]
    [ TO { <replaceable class="parameter">role_name</replaceable> | PUBLIC | CURRENT_ROLE | CURRENT_USER | SESSION_USER } [, ...] ]
    [ USING ( <replaceable class="parameter">using_expression</replaceable> ) ]
    [ WITH CHECK ( <replaceable class="parameter">check_expression</replaceable> ) ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   The <command>CREATE POLICY</command> command defines a new row-level
   security policy for a table.  Note that row-level security must be
   enabled on the table (using <command>ALTER TABLE ... ENABLE ROW LEVEL
   SECURITY</command>) in order for created policies to be applied.
  </para>

  <para>
   A policy grants the permission to select, insert, update, or delete rows
   that match the relevant policy expression.  Existing table rows are
   checked against the expression specified in <literal>USING</literal>,
   while new rows that would be created via <literal>INSERT</literal>
   or <literal>UPDATE</literal> are checked against the expression specified
   in <literal>WITH CHECK</literal>.  When a <literal>USING</literal>
   expression returns true for a given row then that row is visible to the
   user, while if false or null is returned then the row is not visible.
   Typically, no error occurs when a row is not visible, but see
   <xref linkend="sql-createpolicy-summary"/> for exceptions.
   When a <literal>WITH CHECK</literal> expression returns true for a row
   then that row is inserted or updated, while if false or null is returned
   then an error occurs.
  </para>

  <para>
   For <command>INSERT</command>, <command>UPDATE</command>, and
   <command>MERGE</command> statements,
   <literal>WITH CHECK</literal> expressions are enforced after
   <literal>BEFORE</literal> triggers are fired, and before any actual data
   modifications are made.  Thus a <literal>BEFORE ROW</literal> trigger may
   modify the data to be inserted, affecting the result of the security
   policy check.  <literal>WITH CHECK</literal> expressions are enforced
   before any other constraints.
  </para>

  <para>
   Policy names are per-table.  Therefore, one policy name can be used for many
   different tables and have a definition for each table which is appropriate to
   that table.
  </para>

  <para>
   Policies can be applied for specific commands or for specific roles.  The
   default for newly created policies is that they apply for all commands and
   roles, unless otherwise specified.  Multiple policies may apply to a single
   command; see below for more details.
   <xref linkend="sql-createpolicy-summary"/> summarizes how the different types
   of policy apply to specific commands.
  </para>

  <para>
   For policies that can have both <literal>USING</literal>
   and <literal>WITH CHECK</literal> expressions (<literal>ALL</literal>
   and <literal>UPDATE</literal>), if no <literal>WITH CHECK</literal>
   expression is defined, then the <literal>USING</literal> expression will be
   used both to determine which rows are visible (normal
   <literal>USING</literal> case) and which new rows will be allowed to be
   added (<literal>WITH CHECK</literal> case).
  </para>

  <para>
   If row-level security is enabled for a table, but no applicable policies
   exist, a <quote>default deny</quote> policy is assumed, so that no rows will
   be visible or updatable.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">name</replaceable></term>
    <listitem>
     <para>
      The name of the policy to be created.  This must be distinct from the
      name of any other policy for the table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">table_name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of the table the
      policy applies to.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>PERMISSIVE</literal></term>
    <listitem>
     <para>
      Specify that the policy is to be created as a permissive policy.
      All permissive policies which are applicable to a given query will
      be combined together using the Boolean <quote>OR</quote> operator.  By creating
      permissive policies, administrators can add to the set of records
      which can be accessed.  Policies are permissive by default.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>RESTRICTIVE</literal></term>
    <listitem>
     <para>
      Specify that the policy is to be created as a restrictive policy.
      All restrictive policies which are applicable to a given query will
      be combined together using the Boolean <quote>AND</quote> operator.  By creating
      restrictive policies, administrators can reduce the set of records
      which can be accessed as all restrictive policies must be passed for
      each record.
     </para>

     <para>
      Note that there needs to be at least one permissive policy to grant
      access to records before restrictive policies can be usefully used to
      reduce that access. If only restrictive policies exist, then no records
      will be accessible. When a mix of permissive and restrictive policies
      are present, a record is only accessible if at least one of the
      permissive policies passes, in addition to all the restrictive
      policies.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">command</replaceable></term>
    <listitem>
     <para>
      The command to which the policy applies.  Valid options are
      <command>ALL</command>, <command>SELECT</command>,
      <command>INSERT</command>, <command>UPDATE</command>,
      and <command>DELETE</command>.
      <command>ALL</command> is the default.
      See below for specifics regarding how these are applied.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">role_name</replaceable></term>
    <listitem>
     <para>
      The role(s) to which the policy is to be applied.  The default is
      <literal>PUBLIC</literal>, which will apply the policy to all roles.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">using_expression</replaceable></term>
    <listitem>
     <para>
      Any <acronym>SQL</acronym> conditional expression (returning
      <type>boolean</type>).  The conditional expression cannot contain
      any aggregate or window functions.  This expression will be added
      to queries that refer to the table if row-level security is enabled.
      Rows for which the expression returns true will be visible.  Any
      rows for which the expression returns false or null will not be
      visible to the user (in a <command>SELECT</command>), and will not be
      available for modification (in an <command>UPDATE</command>
      or <command>DELETE</command>).  Typically, such rows are silently
      suppressed; no error is reported (but see
      <xref linkend="sql-createpolicy-summary"/> for exceptions).
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">check_expression</replaceable></term>
    <listitem>
     <para>
      Any <acronym>SQL</acronym> conditional expression (returning
      <type>boolean</type>).  The conditional expression cannot contain
      any aggregate or window functions.  This expression will be used in
      <command>INSERT</command> and <command>UPDATE</command> queries against
      the table if row-level security is enabled.  Only rows for which the
      expression evaluates to true will be allowed.  An error will be thrown
      if the expression evaluates to false or null for any of the records
      inserted or any of the records that result from the update.  Note that
      the <replaceable class="parameter">check_expression</replaceable> is
      evaluated against the proposed new contents of the row, not the
      original contents.
     </para>
    </listitem>
   </varlistentry>

  </variablelist>

  <refsect2>
   <title>Per-Command Policies</title>

   <variablelist>

     <varlistentry id="sql-createpolicy-all">
      <term><literal>ALL</literal></term>
      <listitem>
       <para>
         Using <literal>ALL</literal> for a policy means that it will apply
         to all commands, regardless of the type of command.  If an
         <literal>ALL</literal> policy exists and more specific policies
         exist, then both the <literal>ALL</literal> policy and the more
         specific policy (or policies) will be applied.
         Additionally, <literal>ALL</literal> policies will be applied to
         both the selection side of a query and the modification side, using
         the <literal>USING</literal> expression for both cases if only
         a <literal>USING</literal> expression has been defined.
       </para>
       <para>
         As an example, if an <literal>UPDATE</literal> is issued, then the
         <literal>ALL</literal> policy will be applicable both to what the
         <literal>UPDATE</literal> will be able to select as rows to be
         updated (applying the <literal>USING</literal> expression),
         and to the resulting updated rows, to check if they are permitted
         to be added to the table (applying the <literal>WITH CHECK</literal>
         expression, if defined, and the <literal>USING</literal> expression
         otherwise).  If an <command>INSERT</command>
         or <command>UPDATE</command> command attempts to add rows to the
         table that do not pass the <literal>ALL</literal>
         policy's <literal>WITH CHECK</literal> expression (or its
         <literal>USING</literal> expression, if it does not have a
         <literal>WITH CHECK</literal> expression), the entire command will
         be aborted.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-createpolicy-select">
      <term><literal>SELECT</literal></term>
      <listitem>
       <para>
         Using <literal>SELECT</literal> for a policy means that it will apply
         to <literal>SELECT</literal> queries and whenever
         <literal>SELECT</literal> permissions are required on the relation the
         policy is defined for.  The result is that only those records from the
         relation that pass the <literal>SELECT</literal> policy will be
         returned during a <literal>SELECT</literal> query, and that queries
         that require <literal>SELECT</literal> permissions, such as
         <literal>UPDATE</literal>, <literal>DELETE</literal>, and
         <literal>MERGE</literal>, will also only see those records
         that are allowed by the <literal>SELECT</literal> policy.
         A <literal>SELECT</literal> policy cannot have a <literal>WITH
         CHECK</literal> expression, as it only applies in cases where
         records are being retrieved from the relation, except as described
         below.
       </para>
       <para>
         If a data-modifying query has a <literal>RETURNING</literal> clause,
         <literal>SELECT</literal> permissions are required on the relation,
         and any newly inserted or updated rows from the relation must satisfy
         the relation's <literal>SELECT</literal> policies in order to be
         available to the <literal>RETURNING</literal> clause.  If a newly
         inserted or updated row does not satisfy the relation's
         <literal>SELECT</literal> policies, an error will be thrown (inserted
         or updated rows to be returned are <emphasis>never</emphasis>
         silently ignored).
       </para>
       <para>
         If an <literal>INSERT</literal> has an <literal>ON CONFLICT DO
         UPDATE</literal> clause, or an <literal>ON CONFLICT DO
         NOTHING</literal> clause with an arbiter index or constraint
         specification, then <literal>SELECT</literal>
         permissions are required on the relation, and the rows proposed for
         insertion are checked using the relation's <literal>SELECT</literal>
         policies.  If a row proposed for insertion does not satisfy the
         relation's <literal>SELECT</literal> policies, an error is thrown
         (the <literal>INSERT</literal> is <emphasis>never</emphasis> silently
         avoided).  In addition, if the <literal>UPDATE</literal> path is
         taken, the row to be updated and the new updated row are checked
         against the relation's <literal>SELECT</literal> policies, and an
         error is thrown if they are not satisfied (an auxiliary
         <literal>UPDATE</literal> is <emphasis>never</emphasis> silently
         avoided).
       </para>
       <para>
         A <literal>MERGE</literal> command requires <literal>SELECT</literal>
         permissions on both the source and target relations, and so each
         relation's <literal>SELECT</literal> policies are applied before they
         are joined, and the <literal>MERGE</literal> actions will only see
         those records that are allowed by those policies.  In addition, if
         an <literal>UPDATE</literal> action is executed, the target relation's
         <literal>SELECT</literal> policies are applied to the updated row, as
         for a standalone <literal>UPDATE</literal>, except that an error is
         thrown if they are not satisfied.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-createpolicy-insert">
      <term><literal>INSERT</literal></term>
      <listitem>
       <para>
         Using <literal>INSERT</literal> for a policy means that it will apply
         to <literal>INSERT</literal> commands and <literal>MERGE</literal>
         commands that contain <literal>INSERT</literal> actions.
         Rows being inserted that do
         not pass this policy will result in a policy violation error, and the
         entire <literal>INSERT</literal> command will be aborted.
         An <literal>INSERT</literal> policy cannot have
         a <literal>USING</literal> expression, as it only applies in cases
         where records are being added to the relation.
       </para>
       <para>
         Note that an <literal>INSERT</literal> with an <literal>ON CONFLICT
         DO NOTHING/UPDATE</literal> clause will check the
         <literal>INSERT</literal> policies' <literal>WITH CHECK</literal>
         expressions for all rows proposed for insertion, regardless of
         whether or not they end up being inserted.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-createpolicy-update">
      <term><literal>UPDATE</literal></term>
      <listitem>
       <para>
         Using <literal>UPDATE</literal> for a policy means that it will apply
         to <literal>UPDATE</literal>, <literal>SELECT FOR UPDATE</literal>,
         and <literal>SELECT FOR SHARE</literal> commands, as well as
         auxiliary <literal>ON CONFLICT DO UPDATE</literal> clauses of
         <literal>INSERT</literal> commands, and <literal>MERGE</literal>
         commands containing <literal>UPDATE</literal> actions.
         Since an <literal>UPDATE</literal> command
         involves pulling an existing record and replacing it with a new
         modified record, <literal>UPDATE</literal>
         policies accept both a <literal>USING</literal> expression and
         a <literal>WITH CHECK</literal> expression.
         The <literal>USING</literal> expression determines which records
         the <literal>UPDATE</literal> command will see to operate against,
         while the <literal>WITH CHECK</literal> expression defines which
         modified rows are allowed to be stored back into the relation.
       </para>

       <para>
         Any rows whose updated values do not pass the
         <literal>WITH CHECK</literal> expression will cause an error, and the
         entire command will be aborted.  If only a <literal>USING</literal>
         clause is specified, then that clause will be used for both
         <literal>USING</literal> and <literal>WITH CHECK</literal> cases.
       </para>

       <para>
         Typically an <literal>UPDATE</literal> command also needs to read
         data from columns in the relation being updated (e.g., in a
         <literal>WHERE</literal> clause or a <literal>RETURNING</literal>
         clause, or in an expression on the right hand side of the
         <literal>SET</literal> clause).  In this case,
         <literal>SELECT</literal> rights are also required on the relation
         being updated, and the appropriate <literal>SELECT</literal> or
         <literal>ALL</literal> policies will be applied in addition to
         the <literal>UPDATE</literal> policies.  Thus the user must have
         access to the row(s) being updated through a <literal>SELECT</literal>
         or <literal>ALL</literal> policy in addition to being granted
         permission to update the row(s) via an <literal>UPDATE</literal>
         or <literal>ALL</literal> policy.
       </para>

       <para>
         When an <literal>INSERT</literal> command has an auxiliary
         <literal>ON CONFLICT DO UPDATE</literal> clause, if the
         <literal>UPDATE</literal> path is taken, the row to be updated is
         first checked against the <literal>USING</literal> expressions of
         any <literal>UPDATE</literal> policies, and then the new updated row
         is checked against the <literal>WITH CHECK</literal> expressions.
         Note, however, that unlike a standalone <literal>UPDATE</literal>
         command, if the existing row does not pass the
         <literal>USING</literal> expressions, an error will be thrown (the
         <literal>UPDATE</literal> path will <emphasis>never</emphasis> be silently
         avoided).  The same applies to an <literal>UPDATE</literal> action
         of a <command>MERGE</command> command.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-createpolicy-delete">
      <term><literal>DELETE</literal></term>
      <listitem>
       <para>
         Using <literal>DELETE</literal> for a policy means that it will apply
         to <literal>DELETE</literal> commands and <literal>MERGE</literal>
         commands containing <literal>DELETE</literal> actions.  For a
         <literal>DELETE</literal> command, only rows that pass this policy
         will be seen by the <literal>DELETE</literal> command.  There can
         be rows that are visible through a <literal>SELECT</literal> policy
         that are not available for deletion, if they do not pass the
         <literal>USING</literal> expression for the <literal>DELETE</literal>
         policy.  Note, however, that a <literal>DELETE</literal> action in a
         <literal>MERGE</literal> command will see rows that are visible
         through <literal>SELECT</literal> policies, and if the
         <literal>DELETE</literal> policy does not pass for such a row, an
         error will be thrown.
       </para>

       <para>
         In most cases a <literal>DELETE</literal> command also needs to read
         data from columns in the relation that it is deleting from (e.g.,
         in a <literal>WHERE</literal> clause or a
         <literal>RETURNING</literal> clause). In this case,
         <literal>SELECT</literal> rights are also required on the relation,
         and the appropriate <literal>SELECT</literal> or
         <literal>ALL</literal> policies will be applied in addition to
         the <literal>DELETE</literal> policies.  Thus the user must have
         access to the row(s) being deleted through a <literal>SELECT</literal>
         or <literal>ALL</literal> policy in addition to being granted
         permission to delete the row(s) via a <literal>DELETE</literal> or
         <literal>ALL</literal> policy.
       </para>

       <para>
         A <literal>DELETE</literal> policy cannot have a <literal>WITH
         CHECK</literal> expression, as it only applies in cases where
         records are being deleted from the relation, so that there is no
         new row to check.
       </para>
      </listitem>
     </varlistentry>

   </variablelist>

   <para>
    <xref linkend="sql-createpolicy-summary"/> summarizes how the different
    types of policy apply to specific commands.  In the table,
    <quote>check</quote> means that the policy expression is checked and an
    error is thrown if it returns false or null, whereas <quote>filter</quote>
    means that the row is silently ignored if the policy expression returns
    false or null.
   </para>

   <table id="sql-createpolicy-summary">
    <title>Policies Applied by Command Type</title>
    <tgroup cols="6">
     <colspec colnum="4" colname="update-using"/>
     <colspec colnum="5" colname="update-check"/>
     <spanspec namest="update-using" nameend="update-check" spanname="update"/>
     <thead>
      <row>
       <entry morerows="1">Command</entry>
       <entry><literal>SELECT/ALL policy</literal></entry>
       <entry><literal>INSERT/ALL policy</literal></entry>
       <entry spanname="update"><literal>UPDATE/ALL policy</literal></entry>
       <entry><literal>DELETE/ALL policy</literal></entry>
      </row>
      <row>
       <entry><literal>USING expression</literal></entry>
       <entry><literal>WITH CHECK expression</literal></entry>
       <entry><literal>USING expression</literal></entry>
       <entry><literal>WITH CHECK expression</literal></entry>
       <entry><literal>USING expression</literal></entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry><command>SELECT</command> / <command>COPY ... TO</command></entry>
       <entry>Filter existing row</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
      </row>
      <row>
       <entry><command>SELECT FOR UPDATE/SHARE</command></entry>
       <entry>Filter existing row</entry>
       <entry>&mdash;</entry>
       <entry>Filter existing row</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
      </row>
      <row>
       <entry><command>INSERT</command></entry>
       <entry>
        Check new row&nbsp;<footnote id="rls-select-priv">
         <para>
          If read access is required to either the existing or new row (for
          example, a <literal>WHERE</literal> or <literal>RETURNING</literal>
          clause that refers to columns from the relation).
         </para>
        </footnote>
       </entry>
       <entry>Check new row</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
      </row>
      <row>
       <entry><command>UPDATE</command></entry>
       <entry>
        Filter existing row&nbsp;<footnoteref linkend="rls-select-priv"/> &amp;
        check new row&nbsp;<footnoteref linkend="rls-select-priv"/>
       </entry>
       <entry>&mdash;</entry>
       <entry>Filter existing row</entry>
       <entry>Check new row</entry>
       <entry>&mdash;</entry>
      </row>
      <row>
       <entry><command>DELETE</command></entry>
       <entry>
        Filter existing row&nbsp;<footnoteref linkend="rls-select-priv"/>
       </entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>Filter existing row</entry>
      </row>
      <row>
       <entry><command>INSERT ... ON CONFLICT</command></entry>
       <entry>
        Check new row&nbsp;<footnote>
         <para>
          If an arbiter index or constraint is specified.
         </para>
        </footnote><footnote id="rls-on-conflict-priv">
         <para>
          Row proposed for insertion is checked regardless of whether or not a
          conflict occurs.
         </para>
        </footnote>
       </entry>
       <entry>
        Check new row&nbsp;<footnoteref linkend="rls-on-conflict-priv"/>
       </entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
      </row>
      <row>
       <entry><command>ON CONFLICT DO UPDATE</command></entry>
       <entry>
        Check existing &amp; new rows&nbsp;<footnote id="rls-on-conflict-update-priv">
         <para>
          New row of the auxiliary <command>UPDATE</command> command, which
          might be different from the new row of the original
          <command>INSERT</command> command.
         </para>
        </footnote>
       </entry>
       <entry>&mdash;</entry>
       <entry>Check existing row</entry>
       <entry>
        Check new row&nbsp;<footnoteref linkend="rls-on-conflict-update-priv"/>
       </entry>
       <entry>&mdash;</entry>
      </row>
      <row>
       <entry><command>MERGE</command></entry>
       <entry>Filter source &amp; target rows</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
      </row>
      <row>
       <entry><command>MERGE ... THEN INSERT</command></entry>
       <entry>
        Check new row&nbsp;<footnoteref linkend="rls-select-priv"/>
       </entry>
       <entry>Check new row</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
      </row>
      <row>
       <entry><command>MERGE ... THEN UPDATE</command></entry>
       <entry>Check new row</entry>
       <entry>&mdash;</entry>
       <entry>Check existing row</entry>
       <entry>Check new row</entry>
       <entry>&mdash;</entry>
      </row>
      <row>
       <entry><command>MERGE ... THEN DELETE</command></entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>&mdash;</entry>
       <entry>Check existing row</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  </refsect2>

  <refsect2>
   <title>Application of Multiple Policies</title>

   <para>
    When multiple policies of different command types apply to the same command
    (for example, <literal>SELECT</literal> and <literal>UPDATE</literal>
    policies applied to an <literal>UPDATE</literal> command), then the user
    must have both types of permissions (for example, permission to select rows
    from the relation as well as permission to update them).  Thus the
    expressions for one type of policy are combined with the expressions for
    the other type of policy using the <literal>AND</literal> operator.
   </para>

   <para>
    When multiple policies of the same command type apply to the same command,
    then there must be at least one <literal>PERMISSIVE</literal> policy
    granting access to the relation, and all of the
    <literal>RESTRICTIVE</literal> policies must pass.  Thus all the
    <literal>PERMISSIVE</literal> policy expressions are combined using
    <literal>OR</literal>, all the <literal>RESTRICTIVE</literal> policy
    expressions are combined using <literal>AND</literal>, and the results are
    combined using <literal>AND</literal>.  If there are no
    <literal>PERMISSIVE</literal> policies, then access is denied.
   </para>

   <para>
    Note that, for the purposes of combining multiple policies,
    <literal>ALL</literal> policies are treated as having the same type as
    whichever other type of policy is being applied.
   </para>

   <para>
    For example, in an <literal>UPDATE</literal> command requiring both
    <literal>SELECT</literal> and <literal>UPDATE</literal> permissions, if
    there are multiple applicable policies of each type, they will be combined
    as follows:

<programlisting>
<replaceable>expression</replaceable> from RESTRICTIVE SELECT/ALL policy 1
AND
<replaceable>expression</replaceable> from RESTRICTIVE SELECT/ALL policy 2
AND
...
AND
(
  <replaceable>expression</replaceable> from PERMISSIVE SELECT/ALL policy 1
  OR
  <replaceable>expression</replaceable> from PERMISSIVE SELECT/ALL policy 2
  OR
  ...
)
AND
<replaceable>expression</replaceable> from RESTRICTIVE UPDATE/ALL policy 1
AND
<replaceable>expression</replaceable> from RESTRICTIVE UPDATE/ALL policy 2
AND
...
AND
(
  <replaceable>expression</replaceable> from PERMISSIVE UPDATE/ALL policy 1
  OR
  <replaceable>expression</replaceable> from PERMISSIVE UPDATE/ALL policy 2
  OR
  ...
)
</programlisting></para>

  </refsect2>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   You must be the owner of a table to create or change policies for it.
  </para>

  <para>
   While policies will be applied for explicit queries against tables
   in the database, they are not applied when the system is performing internal
   referential integrity checks or validating constraints.  This means there are
   indirect ways to determine that a given value exists.  An example of this is
   attempting to insert a duplicate value into a column that is a primary key
   or has a unique constraint.  If the insert fails then the user can infer that
   the value already exists. (This example assumes that the user is permitted by
   policy to insert records which they are not allowed to see.)  Another example
   is where a user is allowed to insert into a table which references another,
   otherwise hidden table.  Existence can be determined by the user inserting
   values into the referencing table, where success would indicate that the
   value exists in the referenced table.  These issues can be addressed by
   carefully crafting policies to prevent users from being able to insert,
   delete, or update records at all which might possibly indicate a value they
   are not otherwise able to see, or by using generated values (e.g., surrogate
   keys) instead of keys with external meanings.
  </para>

  <para>
   Generally, the system will enforce filter conditions imposed using
   security policies prior to qualifications that appear in user queries,
   in order to prevent inadvertent exposure of the protected data to
   user-defined functions which might not be trustworthy.  However,
   functions and operators marked by the system (or the system
   administrator) as <literal>LEAKPROOF</literal> may be evaluated before
   policy expressions, as they are assumed to be trustworthy.
  </para>

  <para>
   Since policy expressions
   are added to the user's query directly, they will be run with the rights of
   the user running the overall query.  Therefore, users who are using a given
   policy must be able to access any tables or functions referenced in the
   expression or they will simply receive a permission denied error when
   attempting to query the table that has row-level security enabled.
   This does not change how views
   work, however.  As with normal queries and views, permission checks and
   policies for the tables which are referenced by a view will use the view
   owner's rights and any policies which apply to the view owner, except if
   the view is defined using the <literal>security_invoker</literal> option
   (see <link linkend="sql-createview"><command>CREATE VIEW</command></link>).
  </para>

  <para>
   No separate policy exists for <command>MERGE</command>. Instead, the policies
   defined for <command>SELECT</command>, <command>INSERT</command>,
   <command>UPDATE</command>, and <command>DELETE</command> are applied
   while executing <command>MERGE</command>, depending on the actions that are
   performed.
  </para>

  <para>
   Additional discussion and practical examples can be found
   in <xref linkend="ddl-rowsecurity"/>.
  </para>

 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   <command>CREATE POLICY</command> is a <productname>PostgreSQL</productname>
   extension.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-alterpolicy"/></member>
   <member><xref linkend="sql-droppolicy"/></member>
   <member><xref linkend="sql-altertable"/></member>
  </simplelist>
 </refsect1>

</refentry>
