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

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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.
This commit is contained in:
Tom Lane
2010-11-07 13:03:19 -05:00
parent e43fb604d6
commit 543d22fc74
6 changed files with 126 additions and 11 deletions
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27
doc/src/sgml/ref/create_cast.sgml
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@@ -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>