Prevent invoking I/O conversion casts via functional/attribute notation.

PG 8.4 added a built-in feature for casting pretty much any data type to
string types (text, varchar, etc).  We allowed this to work in any of the
historically-allowed syntaxes: CAST(x AS text), x::text, text(x), or
x.text.  However, multiple complaints have shown that it's too easy to
invoke such casts unintentionally in the latter two styles, particularly
field selection.  To cure the problem with the narrowest possible change
of behavior, disallow use of I/O conversion casts from composite types to
string types via functional/attribute syntax.  The new functionality is
still available via cast syntax.

In passing, document the equivalence of functional and attribute syntax
in a more visible place.

doc/src/sgml/ref/create_cast.sgml

@@ -38,7 +38,7 @@ CREATE CAST (<replaceable>source_type</replaceable> AS <replaceable>target_type<
   <para>
    <command>CREATE CAST</command> defines a new cast.  A cast
    specifies how to perform a conversion between
-   two data types.  For example:
+   two data types.  For example,
 <programlisting>
 SELECT CAST(42 AS float8);
 </programlisting>
@@ -64,10 +64,13 @@ SELECT CAST(42 AS float8);
   </para>
 
   <para>
-   You can define a cast as an <firstterm>I/O conversion cast</> using
+   You can define a cast as an <firstterm>I/O conversion cast</> by using
    the <literal>WITH INOUT</literal> syntax. An I/O conversion cast is
    performed by invoking the output function of the source data type, and
-   passing the result to the input function of the target data type.
+   passing the resulting string to the input function of the target data type.
+   In many common cases, this feature avoids the need to write a separate
+   cast function for conversion. An I/O conversion cast acts the same as
+   a regular function-based cast; only the implementation is different.
   </para>
 
   <para>
@@ -218,7 +221,7 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
       <para>
        Indicates that the cast is an I/O conversion cast, performed by
        invoking the output function of the source data type, and passing the
-       result to the input function of the target data type.
+       resulting string to the input function of the target data type.
       </para>
      </listitem>
     </varlistentry>
@@ -278,9 +281,9 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
   <para>
    When a cast has different source and
    target types and a function that takes more than one argument, it
-   represents converting from one type to another and applying a length
+   supports converting from one type to another and applying a length
    coercion in a single step.  When no such entry is available, coercion
-   to a type that uses a type modifier involves two steps, one to
+   to a type that uses a type modifier involves two cast steps, one to
    convert between data types and a second to apply the modifier.
   </para>
 
@@ -366,6 +369,18 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
     syntax.
    </para>
   </note>
+
+  <note>
+   <para>
+    There's an exception to the exception, too: I/O conversion casts from
+    composite types to string types cannot be invoked using functional
+    syntax, but must be written in explicit cast syntax (either
+    <literal>CAST</> or <literal>::</> notation).  This exception was added
+    because after the introduction of automatically-provided I/O conversion
+    casts, it was found too easy to accidentally invoke such a cast when
+    a function or column reference was intended.
+   </para>
+  </note>
  </refsect1>
 
 

doc/src/sgml/syntax.sgml

@@ -1522,6 +1522,19 @@ sqrt(2)
     The arguments can optionally have names attached.
     See <xref linkend="sql-syntax-calling-funcs"> for details.
    </para>
+
+   <note>
+    <para>
+     A function that takes a single argument of composite type can
+     optionally be called using field-selection syntax, and conversely
+     field selection can be written in functional style.  That is, the
+     notations <literal>col(table)</> and <literal>table.col</> are
+     interchangeable.  This behavior is not SQL-standard but is provided
+     in <productname>PostgreSQL</> because it allows use of functions to
+     emulate <quote>computed fields</>.  For more information see
+     <xref linkend="xfunc-sql-composite-functions">.
+    </para>
+   </note>
   </sect2>
 
   <sect2 id="syntax-aggregates">

doc/src/sgml/xfunc.sgml

@@ -271,7 +271,7 @@ $$ LANGUAGE SQL;
     </para>
    </sect2>
 
-   <sect2>
+   <sect2 id="xfunc-sql-composite-functions">
     <title><acronym>SQL</acronym> Functions on Composite Types</title>
 
     <para>
@@ -492,6 +492,12 @@ SELECT emp.name, emp.double_salary FROM emp;
       <literal>double_salary</> isn't a real column of the table.
       (You can also emulate computed fields with views.)
      </para>
+
+     <para>
+      Because of this behavior, it's unwise to give a function that takes
+      a single composite-type argument the same name as any of the fields of
+      that composite type.
+     </para>
     </tip>
 
     <para>

src/backend/parser/parse_func.c

@@ -985,8 +985,13 @@ func_get_detail(List *funcname,
 		 * can't write "foo[] (something)" as a function call.  In theory
 		 * someone might want to invoke it as "_foo (something)" but we have
 		 * never supported that historically, so we can insist that people
-		 * write it as a normal cast instead.  Lack of historical support is
-		 * also the reason for not considering composite-type casts here.
+		 * write it as a normal cast instead.
+		 *
+		 * We also reject the specific case of COERCEVIAIO for a composite
+		 * source type and a string-category target type.  This is a case that
+		 * find_coercion_pathway() allows by default, but experience has shown
+		 * that it's too commonly invoked by mistake.  So, again, insist that
+		 * people use cast syntax if they want to do that.
 		 *
 		 * NB: it's important that this code does not exceed what coerce_type
 		 * can do, because the caller will try to apply coerce_type if we
@@ -1017,8 +1022,23 @@ func_get_detail(List *funcname,
 					cpathtype = find_coercion_pathway(targetType, sourceType,
 													  COERCION_EXPLICIT,
 													  &cfuncid);
-					iscoercion = (cpathtype == COERCION_PATH_RELABELTYPE ||
-								  cpathtype == COERCION_PATH_COERCEVIAIO);
+					switch (cpathtype)
+					{
+						case COERCION_PATH_RELABELTYPE:
+							iscoercion = true;
+							break;
+						case COERCION_PATH_COERCEVIAIO:
+							if ((sourceType == RECORDOID ||
+								 ISCOMPLEX(sourceType)) &&
+								TypeCategory(targetType) == TYPCATEGORY_STRING)
+								iscoercion = false;
+							else
+								iscoercion = true;
+							break;
+						default:
+							iscoercion = false;
+							break;
+					}
 				}
 
 				if (iscoercion)

src/test/regress/expected/rowtypes.out

@@ -324,3 +324,49 @@ select * from price;
 (3 rows)
 
 rollback;
+--
+-- We allow I/O conversion casts from composite types to strings to be
+-- invoked via cast syntax, but not functional syntax.  This is because
+-- the latter is too prone to be invoked unintentionally.
+--
+select cast (fullname as text) from fullname;
+ fullname 
+----------
+(0 rows)
+
+select fullname::text from fullname;
+ fullname 
+----------
+(0 rows)
+
+select text(fullname) from fullname;  -- error
+ERROR:  function text(fullname) does not exist
+LINE 1: select text(fullname) from fullname;
+               ^
+HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
+select fullname.text from fullname;  -- error
+ERROR:  column fullname.text does not exist
+LINE 1: select fullname.text from fullname;
+               ^
+-- same, but RECORD instead of named composite type:
+select cast (row('Jim', 'Beam') as text);
+    row     
+------------
+ (Jim,Beam)
+(1 row)
+
+select (row('Jim', 'Beam'))::text;
+    row     
+------------
+ (Jim,Beam)
+(1 row)
+
+select text(row('Jim', 'Beam'));  -- error
+ERROR:  function text(record) does not exist
+LINE 1: select text(row('Jim', 'Beam'));
+               ^
+HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
+select (row('Jim', 'Beam')).text;  -- error
+ERROR:  could not identify column "text" in record data type
+LINE 1: select (row('Jim', 'Beam')).text;
+                ^

src/test/regress/sql/rowtypes.sql

@@ -157,3 +157,18 @@ UPDATE price
 select * from price;
 
 rollback;
+
+--
+-- We allow I/O conversion casts from composite types to strings to be
+-- invoked via cast syntax, but not functional syntax.  This is because
+-- the latter is too prone to be invoked unintentionally.
+--
+select cast (fullname as text) from fullname;
+select fullname::text from fullname;
+select text(fullname) from fullname;  -- error
+select fullname.text from fullname;  -- error
+-- same, but RECORD instead of named composite type:
+select cast (row('Jim', 'Beam') as text);
+select (row('Jim', 'Beam'))::text;
+select text(row('Jim', 'Beam'));  -- error
+select (row('Jim', 'Beam')).text;  -- error