database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-08-31 17:02:12 +03:00
Code Activity

Downgrade implicit casts to text to be assignment-only, except for the ones

Browse Source 
from the other string-category types; this eliminates a lot of surprising
interpretations that the parser could formerly make when there was no directly
applicable operator.

Create a general mechanism that supports casts to and from the standard string
types (text,varchar,bpchar) for *every* datatype, by invoking the datatype's
I/O functions.  These new casts are assignment-only in the to-string direction,
explicit-only in the other, and therefore should create no surprising behavior.
Remove a bunch of thereby-obsoleted datatype-specific casting functions.

The "general mechanism" is a new expression node type CoerceViaIO that can
actually convert between *any* two datatypes if their external text
representations are compatible.  This is more general than needed for the
immediate feature, but might be useful in plpgsql or other places in future.

This commit does nothing about the issue that applying the concatenation
operator || to non-text types will now fail, often with strange error messages
due to misinterpreting the operator as array concatenation.  Since it often
(not always) worked before, we should either make it succeed or at least give
a more user-friendly error; but details are still under debate.

Peter Eisentraut and Tom Lane
This commit is contained in:
Tom Lane
2007-06-05 21:31:09 +00:00
parent 1120b99445
commit 31edbadf4a
60 changed files with 850 additions and 1612 deletions
Download Patch File Download Diff File Expand all files Collapse all files

39
doc/src/sgml/syntax.sgml
View File

@@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.118 2007/05/11 17:57:11 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.119 2007/06/05 21:31:04 tgl Exp $ -->
<chapter id="sql-syntax">
<title>SQL Syntax</title>
@@ -561,18 +561,18 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
The <literal>::</literal>, <literal>CAST()</literal>, and
function-call syntaxes can also be used to specify run-time type
conversions of arbitrary expressions, as discussed in <xref
linkend="sql-syntax-type-casts">. But the form
<literal><replaceable>type</replaceable> '<replaceable>string</replaceable>'</literal>
can only be used to specify the type of a literal constant.
Another restriction on
<literal><replaceable>type</replaceable> '<replaceable>string</replaceable>'</literal>
is that it does not work for array types; use <literal>::</literal>
linkend="sql-syntax-type-casts">. To avoid syntactic ambiguity, the
<literal><replaceable>type</> '<replaceable>string</>'</literal>
syntax can only be used to specify the type of a simple literal constant.
Another restriction on the
<literal><replaceable>type</> '<replaceable>string</>'</literal>
syntax is that it does not work for array types; use <literal>::</literal>
or <literal>CAST()</literal> to specify the type of an array constant.
</para>
<para>
The <literal>CAST()</> syntax conforms to SQL. The
<literal><replaceable>type</replaceable> '<replaceable>string</replaceable>'</literal>
<literal><replaceable>type</> '<replaceable>string</>'</literal>
syntax is a generalization of the standard: SQL specifies this syntax only
for a few data types, but <productname>PostgreSQL</productname> allows it
for all types. The syntax with
@@ -1431,16 +1431,21 @@ CAST ( <replaceable>expression</replaceable> AS <replaceable>type</replaceable>
double-quoted, because of syntactic conflicts. Therefore, the use of
the function-like cast syntax leads to inconsistencies and should
probably be avoided in new applications.
(The function-like syntax is in fact just a function call. When
one of the two standard cast syntaxes is used to do a run-time
conversion, it will internally invoke a registered function to
perform the conversion. By convention, these conversion functions
have the same name as their output type, and thus the <quote>function-like
syntax</> is nothing more than a direct invocation of the underlying
conversion function. Obviously, this is not something that a portable
application should rely on.)
</para>
<note>
<para>
The function-like syntax is in fact just a function call. When
one of the two standard cast syntaxes is used to do a run-time
conversion, it will internally invoke a registered function to
perform the conversion. By convention, these conversion functions
have the same name as their output type, and thus the <quote>function-like
syntax</> is nothing more than a direct invocation of the underlying
conversion function. Obviously, this is not something that a portable
application should rely on. For further details see
<xref linkend="sql-createcast" endterm="sql-createcast-title">.
</para>
</note>
</sect2>
<sect2 id="sql-syntax-scalar-subqueries">