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Replace the hard-wired type knowledge in TypeCategory() and IsPreferredType()

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with system catalog lookups, as was foreseen to be necessary almost since
their creation.  Instead put the information into two new pg_type columns,
typcategory and typispreferred.  Add support for setting these when
creating a user-defined base type.

The category column is just a "char" (i.e. a poor man's enum), allowing
a crude form of user extensibility of the category list: just use an
otherwise-unused character.  This seems sufficient for foreseen uses,
but we could upgrade to having an actual category catalog someday, if
there proves to be a huge demand for custom type categories.

In this patch I have attempted to hew exactly to the behavior of the
previous hardwired logic, except for introducing new type categories for
arrays, composites, and enums.  In particular the default preferred state
for user-defined types remains TRUE.  That seems worth revisiting, but it
should be done as a separate patch from introducing the infrastructure.
Likewise, any adjustment of the standard set of categories should be done
separately.
This commit is contained in:
Tom Lane
2008-07-30 17:05:05 +00:00
parent ab9907f5e5
commit bac3e83622
20 changed files with 617 additions and 497 deletions
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50
doc/src/sgml/ref/create_type.sgml
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@@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/create_type.sgml,v 1.74 2008/05/27 18:05:13 tgl Exp $
$PostgreSQL: pgsql/doc/src/sgml/ref/create_type.sgml,v 1.75 2008/07/30 17:05:04 tgl Exp $
PostgreSQL documentation
-->
@@ -38,6 +38,8 @@ CREATE TYPE <replaceable class="parameter">name</replaceable> (
[ , PASSEDBYVALUE ]
[ , ALIGNMENT = <replaceable class="parameter">alignment</replaceable> ]
[ , STORAGE = <replaceable class="parameter">storage</replaceable> ]
[ , CATEGORY = <replaceable class="parameter">category</replaceable> ]
[ , PREFERRED = <replaceable class="parameter">preferred</replaceable> ]
[ , DEFAULT = <replaceable class="parameter">default</replaceable> ]
[ , ELEMENT = <replaceable class="parameter">element</replaceable> ]
[ , DELIMITER = <replaceable class="parameter">delimiter</replaceable> ]
@@ -281,6 +283,27 @@ CREATE TYPE <replaceable class="parameter">name</replaceable>
<literal>external</literal> items.)
</para>
<para>
The <replaceable class="parameter">category</replaceable> and
<replaceable class="parameter">preferred</replaceable> parameters can be
used to help control which implicit cast will be applied in ambiguous
situations. Each data type belongs to a category named by a single ASCII
character, and each type is either <quote>preferred</> or not within its
category. The parser will prefer casting to preferred types (but only from
other types within the same category) when this rule is helpful in
resolving overloaded functions or operators. For more details see <xref
linkend="typeconv">. For types that have no implicit casts to or from any
other types, it is sufficient to leave these settings at the defaults.
However, for a group of related types that have implicit casts, it is often
helpful to mark them all as belonging to a category and select one or two
of the <quote>most general</> types as being preferred within the category.
The <replaceable class="parameter">category</replaceable> parameter is
especially useful when adding a user-defined type to an existing built-in
category, such as the numeric or string types. However, it is also
possible to create new entirely-user-defined type categories. Select any
ASCII character other than an upper-case letter to name such a category.
</para>
<para>
A default value can be specified, in case a user wants columns of the
data type to default to something other than the null value.
@@ -494,6 +517,31 @@ CREATE TYPE <replaceable class="parameter">name</replaceable>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">category</replaceable></term>
<listitem>
<para>
The category code (a single ASCII character) for this type.
The default is <literal>'U'</> for <quote>user-defined type</>.
Other standard category codes can be found in
<xref linkend="catalog-typcategory-table">. You may also choose
other ASCII characters in order to create custom categories.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">preferred</replaceable></term>
<listitem>
<para>
True if this type is a preferred type within its type category,
else false. The default is true (which is appropriate for
all entries in category <literal>U</>, but is usually not
appropriate for new types in other categories &mdash; beware!).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">default</replaceable></term>
<listitem>