Add strict_multi_assignment and too_many_rows plpgsql checks

Until now shadowed_variables was the only plpgsql check supported by
plpgsql.extra_warnings and plpgsql.extra_errors.  This patch introduces
two new checks - strict_multi_assignment and too_many_rows.  Unlike
shadowed_variables, these new checks are enforced at run-time.

strict_multi_assignment checks that commands allowing multi-assignment
(for example SELECT INTO) have the same number of sources and targets.
too_many_rows checks that queries with an INTO clause return one row
exactly.

These checks are aimed at cases that are technically valid and allowed,
but are often a sign of a bug.  Therefore those checks are expected to
be enabled primarily in development and testing environments.

Author: Pavel Stehule
Reviewed-by: Stephen Frost, Tomas Vondra
Discussion: https://www.postgresql.org/message-id/flat/CAFj8pRA2kKRDKpUNwLY0GeG1OqOp+tLS2yQA1V41gzuSz-hCng@mail.gmail.com

doc/src/sgml/plpgsql.sgml

@@ -5034,7 +5034,7 @@ a_output := a_output || $$ if v_$$ || referrer_keys.kind || $$ like '$$
 
   </sect2>
   <sect2 id="plpgsql-extra-checks">
-   <title>Additional Compile-time Checks</title>
+   <title>Additional Compile-time and Run-time Checks</title>
 
    <para>
     To aid the user in finding instances of simple but common problems before
@@ -5046,26 +5046,64 @@ a_output := a_output || $$ if v_$$ || referrer_keys.kind || $$ like '$$
     so you are advised to test in a separate development environment.
    </para>
 
- <para>
-  These additional checks are enabled through the configuration variables
-  <varname>plpgsql.extra_warnings</varname> for warnings and
-  <varname>plpgsql.extra_errors</varname> for errors. Both can be set either to
-  a comma-separated list of checks, <literal>"none"</literal> or <literal>"all"</literal>.
-  The default is <literal>"none"</literal>. Currently the list of available checks
-  includes only one:
-  <variablelist>
-   <varlistentry>
-    <term><varname>shadowed_variables</varname></term>
-    <listitem>
-     <para>
-      Checks if a declaration shadows a previously defined variable.
-     </para>
-    </listitem>
-   </varlistentry>
-  </variablelist>
+   <para>
+    Setting <varname>plpgsql.extra_warnings</varname>, or
+    <varname>plpgsql.extra_errors</varname>, as appropriate, to <literal>"all"</literal>
+    is encouraged in development and/or testing environments.
+   </para>
 
-  The following example shows the effect of <varname>plpgsql.extra_warnings</varname>
-  set to <varname>shadowed_variables</varname>:
+   <para>
+    These additional checks are enabled through the configuration variables
+    <varname>plpgsql.extra_warnings</varname> for warnings and
+    <varname>plpgsql.extra_errors</varname> for errors. Both can be set either to
+    a comma-separated list of checks, <literal>"none"</literal> or
+    <literal>"all"</literal>. The default is <literal>"none"</literal>. Currently
+    the list of available checks includes:
+    <variablelist>
+     <varlistentry>
+      <term><varname>shadowed_variables</varname></term>
+      <listitem>
+       <para>
+        Checks if a declaration shadows a previously defined variable.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term><varname>strict_multi_assignment</varname></term>
+      <listitem>
+       <para>
+        Some <application>PL/PgSQL</application> commands allow assigning
+        values to more than one variable at a time, such as
+        <command>SELECT INTO</command>.  Typically, the number of target
+        variables and the number of source variables should match, though
+        <application>PL/PgSQL</application> will use <literal>NULL</literal>
+        for missing values and extra variables are ignored.  Enabling this
+        check will cause <application>PL/PgSQL</application> to throw a
+        <literal>WARNING</literal> or <literal>ERROR</literal> whenever the
+        number of target variables and the number of source variables are
+        different.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term><varname>too_many_rows</varname></term>
+      <listitem>
+       <para>
+        Enabling this check will cause <application>PL/PgSQL</application> to
+        check if a given query returns more than one row when an
+        <literal>INTO</literal> clause is used.  As an <literal>INTO</literal>
+        statement will only ever use one row, having a query return multiple
+        rows is generally either inefficient and/or nondeterministic and
+        therefore is likely an error.
+       </para>
+      </listitem>
+     </varlistentry>
+    </variablelist>
+
+    The following example shows the effect of <varname>plpgsql.extra_warnings</varname>
+    set to <varname>shadowed_variables</varname>:
 <programlisting>
 SET plpgsql.extra_warnings TO 'shadowed_variables';
 
@@ -5081,8 +5119,41 @@ LINE 3: f1 int;
         ^
 CREATE FUNCTION
 </programlisting>
- </para>
- </sect2>
+    The below example shows the effects of setting
+    <varname>plpgsql.extra_warnings</varname> to
+    <varname>strict_multi_assignment</varname>:
+<programlisting>
+SET plpgsql.extra_warnings TO 'strict_multi_assignment';
+
+CREATE OR REPLACE FUNCTION public.foo()
+ RETURNS void
+ LANGUAGE plpgsql
+AS $$
+DECLARE
+  x int;
+  y int;
+BEGIN
+  SELECT 1 INTO x, y;
+  SELECT 1, 2 INTO x, y;
+  SELECT 1, 2, 3 INTO x, y;
+END;
+$$;
+
+SELECT foo();
+WARNING:  number of source and target fields in assignment do not match
+DETAIL:  strict_multi_assignment check of extra_warnings is active.
+HINT:  Make sure the query returns the exact list of columns.
+WARNING:  number of source and target fields in assignment do not match
+DETAIL:  strict_multi_assignment check of extra_warnings is active.
+HINT:  Make sure the query returns the exact list of columns.
+
+ foo 
+-----
+ 
+(1 row)
+</programlisting>
+   </para>
+  </sect2>
  </sect1>
 
   <!-- **** Porting from Oracle PL/SQL **** -->