Allow LEAKPROOF functions for better performance of security views.

We don't normally allow quals to be pushed down into a view created
with the security_barrier option, but functions without side effects
are an exception: they're OK.  This allows much better performance in
common cases, such as when using an equality operator (that might
even be indexable).

There is an outstanding issue here with the CREATE FUNCTION / ALTER
FUNCTION syntax: there's no way to use ALTER FUNCTION to unset the
leakproof flag.  But I'm committing this as-is so that it doesn't
have to be rebased again; we can fix up the grammar in a future
commit.

KaiGai Kohei, with some wordsmithing by me.

2 files changed, 30 insertions, 2 deletions

diff --git a/doc/src/sgml/ref/alter_function.sgml b/doc/src/sgml/ref/alter_function.sgml
index acb3e4fb93f..3ceb6123f4f 100644
--- a/doc/src/sgml/ref/alter_function.sgml
+++ b/doc/src/sgml/ref/alter_function.sgml
@@ -33,7 +33,7 @@ ALTER FUNCTION <replaceable>name</replaceable> ( [ [ <replaceable class="paramet
 <phrase>where <replaceable class="PARAMETER">action</replaceable> is one of:</phrase>
 
     CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT
-    IMMUTABLE | STABLE | VOLATILE
+    IMMUTABLE | STABLE | VOLATILE | LEAKPROOF
     [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER
     COST <replaceable class="parameter">execution_cost</replaceable>
     ROWS <replaceable class="parameter">result_rows</replaceable>
@@ -191,6 +191,17 @@ ALTER FUNCTION <replaceable>name</replaceable> ( [ [ <replaceable class="paramet
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>LEAKPROOF</literal></term>
+    <listitem>
+     <para>
+      Change whether the function is considered leakproof or not.
+      See <xref linkend="sql-createfunction"> for more information about
+      this capability.
+     </para>
+    </listitem>
+   </varlistentry>
+
     <varlistentry>
      <term><literal>COST</literal> <replaceable class="parameter">execution_cost</replaceable></term>
 
diff --git a/doc/src/sgml/ref/create_function.sgml b/doc/src/sgml/ref/create_function.sgml
index 2a87130356e..7df66ab0e08 100644
--- a/doc/src/sgml/ref/create_function.sgml
+++ b/doc/src/sgml/ref/create_function.sgml
@@ -26,7 +26,7 @@ CREATE [ OR REPLACE ] FUNCTION
       | RETURNS TABLE ( <replaceable class="parameter">column_name</replaceable> <replaceable class="parameter">column_type</replaceable> [, ...] ) ]
   { LANGUAGE <replaceable class="parameter">lang_name</replaceable>
     | WINDOW
-    | IMMUTABLE | STABLE | VOLATILE
+    | IMMUTABLE | STABLE | VOLATILE | LEAKPROOF
     | CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT
     | [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER
     | COST <replaceable class="parameter">execution_cost</replaceable>
@@ -325,6 +325,23 @@ CREATE [ OR REPLACE ] FUNCTION
     </varlistentry>
 
     <varlistentry>
+     <term><literal>LEAKPROOF</literal></term>
+     <listitem>
+      <para>
+       <literal>LEAKPROOF</literal> indicates that the function has no side
+       effects.  It reveals no information about its arguments other than by
+       its return value.  For example, a function which throws an error message
+       for some argument values but not others, or which includes the argument
+       values in any error message, is not leakproof.   The query planner may
+       push leakproof functions (but not others) into views created with the
+       <literal>security_barrier</literal> option.  See
+       <xref linkend="sql-createview"> and <xref linkend="rules-privileges">.
+       This option can only be set by the superuser.
+      </para>
+     </listitem>
+    </varlistentry>
+
+    <varlistentry>
      <term><literal>CALLED ON NULL INPUT</literal></term>
      <term><literal>RETURNS NULL ON NULL INPUT</literal></term>
      <term><literal>STRICT</literal></term>