doc: Improve description of RLS policies applied by command type.HEADmaster

On the CREATE POLICY page, the "Policies Applied by Command Type"
table was missing MERGE ... THEN DELETE and some of the policies
applied during INSERT ... ON CONFLICT and MERGE. Fix that, and try to
improve readability by listing the various MERGE cases separately,
rather than together with INSERT/UPDATE/DELETE. Mention COPY ... TO
along with SELECT, since it behaves in the same way. In addition,
document which policy violations cause errors to be thrown, and which
just cause rows to be silently ignored.

Also, a paragraph above the table states that INSERT ... ON CONFLICT
DO UPDATE only checks the WITH CHECK expressions of INSERT policies
for rows appended to the relation by the INSERT path, which is
incorrect -- all rows proposed for insertion are checked, regardless
of whether they end up being inserted. Fix that, and also mention that
the same applies to INSERT ... ON CONFLICT DO NOTHING.

In addition, in various other places on that page, clarify how the
different types of policy are applied to different commands, and
whether or not errors are thrown when policy checks do not pass.

Backpatch to all supported versions. Prior to v17, MERGE did not
support RETURNING, and so MERGE ... THEN INSERT would never check new
rows against SELECT policies. Prior to v15, MERGE was not supported at
all.

Author: Dean Rasheed <[email protected]>
Reviewed-by: Viktor Holmberg <[email protected]>
Reviewed-by: Jian He <[email protected]>
Discussion: https://postgr.es/m/CAEZATCWqnfeChjK=n1V_dYZT4rt4mnq+ybf9c0qXDYTVMsy8pg@mail.gmail.com
Backpatch-through: 14

1 files changed, 159 insertions, 44 deletions

diff --git a/doc/src/sgml/ref/create_policy.sgml b/doc/src/sgml/ref/create_policy.sgml
index e76c342d3da..42d43ad7bf4 100644
--- a/doc/src/sgml/ref/create_policy.sgml
+++ b/doc/src/sgml/ref/create_policy.sgml
@@ -49,6 +49,8 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
    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.
@@ -194,8 +196,9 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
       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>).  Such rows are silently suppressed; no error
-      is reported.
+      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>
@@ -251,8 +254,10 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
          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, the entire
-         command will be aborted.
+         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>
@@ -268,11 +273,50 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
          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>, will also only see those records
+         <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.
+         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
+         NOTHING/UPDATE</literal> clause, <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>
@@ -292,10 +336,11 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
          where records are being added to the relation.
        </para>
        <para>
-         Note that <literal>INSERT</literal> with <literal>ON CONFLICT DO
-         UPDATE</literal> checks <literal>INSERT</literal> policies'
-         <literal>WITH CHECK</literal> expressions only for rows appended
-         to the relation by the <literal>INSERT</literal> path.
+         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>
@@ -305,12 +350,12 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
       <listitem>
        <para>
          Using <literal>UPDATE</literal> for a policy means that it will apply
-         to <literal>UPDATE</literal>, <literal>SELECT FOR UPDATE</literal>
+         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.
-         <literal>MERGE</literal> commands containing <literal>UPDATE</literal>
-         actions are affected as well.  Since <literal>UPDATE</literal>
+         <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
@@ -356,7 +401,8 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
          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).
+         avoided).  The same applies to an <literal>UPDATE</literal> action
+         of a <command>MERGE</command> command.
        </para>
       </listitem>
      </varlistentry>
@@ -366,12 +412,18 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
       <listitem>
        <para>
          Using <literal>DELETE</literal> for a policy means that it will apply
-         to <literal>DELETE</literal> commands.  Only rows that pass this
-         policy will be seen by a <literal>DELETE</literal> command.  There can
-         be rows that are visible through a <literal>SELECT</literal> that are
-         not available for deletion, if they do not pass the
-         <literal>USING</literal> expression for
-         the <literal>DELETE</literal> policy.
+         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>
@@ -400,6 +452,15 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
 
    </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">
@@ -424,8 +485,8 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
      </thead>
      <tbody>
       <row>
-       <entry><command>SELECT</command></entry>
-       <entry>Existing row</entry>
+       <entry><command>SELECT</command> / <command>COPY ... TO</command></entry>
+       <entry>Filter existing row</entry>
        <entry>&mdash;</entry>
        <entry>&mdash;</entry>
        <entry>&mdash;</entry>
@@ -433,63 +494,117 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
       </row>
       <row>
        <entry><command>SELECT FOR UPDATE/SHARE</command></entry>
-       <entry>Existing row</entry>
+       <entry>Filter existing row</entry>
        <entry>&mdash;</entry>
-       <entry>Existing row</entry>
+       <entry>Filter existing row</entry>
        <entry>&mdash;</entry>
        <entry>&mdash;</entry>
       </row>
       <row>
-       <entry><command>INSERT</command> / <command>MERGE ... THEN INSERT</command></entry>
+       <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>New row</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>INSERT ... RETURNING</command></entry>
+       <entry><command>DELETE</command></entry>
        <entry>
-        New row <footnote id="rls-select-priv">
+        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 id="rls-on-conflict-priv">
          <para>
-          If read access is required to the existing or new row (for example,
-          a <literal>WHERE</literal> or <literal>RETURNING</literal> clause
-          that refers to columns from the relation).
+          Row proposed for insertion is checked regardless of whether or not a
+          conflict occurs.
          </para>
         </footnote>
        </entry>
-       <entry>New row</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>UPDATE</command> / <command>MERGE ... THEN UPDATE</command></entry>
+       <entry><command>ON CONFLICT DO UPDATE</command></entry>
        <entry>
-        Existing &amp; new rows <footnoteref linkend="rls-select-priv"/>
+        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>Existing row</entry>
-       <entry>New row</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>DELETE</command></entry>
+       <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>
-        Existing row <footnoteref linkend="rls-select-priv"/>
+        Check new row&nbsp;<footnoteref linkend="rls-select-priv"/>
        </entry>
+       <entry>Check new row</entry>
        <entry>&mdash;</entry>
        <entry>&mdash;</entry>
        <entry>&mdash;</entry>
-       <entry>Existing row</entry>
       </row>
       <row>
-       <entry><command>ON CONFLICT DO UPDATE</command></entry>
-       <entry>Existing &amp; new rows</entry>
+       <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>Existing row</entry>
-       <entry>New row</entry>
        <entry>&mdash;</entry>
+       <entry>Check existing row</entry>
       </row>
      </tbody>
     </tgroup>