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Add to_regtypemod function to extract typemod from a string type name.

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In combination with to_regtype, this allows converting a string to
the "canonicalized" form emitted by format_type.  That usage requires
parsing the string twice, which is slightly annoying but not really
too expensive.  We considered alternatives such as returning a record
type, but that way was notationally uglier than this, and possibly
less flexible.

Like to_regtype(), we'd rather that this return NULL for any bad
input, but the underlying type-parsing logic isn't yet capable of
not throwing syntax errors.  Adjust the documentation for both
functions to point that out.

In passing, fix up a couple of nearby entries in the System Catalog
Information Functions table that had not gotten the word about our
since-v13 convention for displaying function usage examples.

David Wheeler and Erik Wienhold, reviewed by Pavel Stehule, Jim Jones,
and others.

Discussion: https://postgr.es/m/DF2324CA-2673-4ABE-B382-26B5770B6AA3@justatheory.com
This commit is contained in:
Tom Lane
2024-03-20 17:11:23 -04:00
parent 80686761c4
commit 1218ca9956
6 changed files with 115 additions and 32 deletions
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77
doc/src/sgml/func.sgml
View File

@ -24872,7 +24872,7 @@ SELECT pg_type_is_visible('myschema.widget'::regtype);
<tbody>
<row>
<entry role="func_table_entry"><para role="func_signature">
<entry id="format-type" xreflabel="format_type" role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>format_type</primary>
</indexterm>
@ -25387,18 +25387,8 @@ SELECT currval(pg_get_serial_sequence('sometable', 'id'));
OID for comparison purposes but displays as a type name.
</para>
<para>
For example:
<programlisting>
SELECT pg_typeof(33);
pg_typeof
-----------
integer
SELECT typlen FROM pg_type WHERE oid = pg_typeof(33);
typlen
--------
4
</programlisting>
<literal>pg_typeof(33)</literal>
<returnvalue>integer</returnvalue>
</para></entry>
</row>
@ -25418,18 +25408,12 @@ SELECT typlen FROM pg_type WHERE oid = pg_typeof(33);
collatable data type, then an error is raised.
</para>
<para>
For example:
<programlisting>
SELECT collation for (description) FROM pg_description LIMIT 1;
pg_collation_for
------------------
"default"
SELECT collation for ('foo' COLLATE "de_DE");
pg_collation_for
------------------
"de_DE"
</programlisting>
<literal>collation for ('foo'::text)</literal>
<returnvalue>"default"</returnvalue>
</para>
<para>
<literal>collation for ('foo' COLLATE "de_DE")</literal>
<returnvalue>"de_DE"</returnvalue>
</para></entry>
</row>
@ -25570,7 +25554,7 @@ SELECT collation for ('foo' COLLATE "de_DE");
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<entry id="to-regtype" xreflabel="to_regtype" role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>to_regtype</primary>
</indexterm>
@ -25578,11 +25562,42 @@ SELECT collation for ('foo' COLLATE "de_DE");
<returnvalue>regtype</returnvalue>
</para>
<para>
Translates a textual type name to its OID. A similar result is
obtained by casting the string to type <type>regtype</type> (see
<xref linkend="datatype-oid"/>); however, this function will return
<literal>NULL</literal> rather than throwing an error if the name is
not found.
Parses a string of text, extracts a potential type name from it,
and translates that name into a type OID. A syntax error in the
string will result in an error; but if the string is a
syntactically valid type name that happens not to be found in the
catalogs, the result is <literal>NULL</literal>. A similar result
is obtained by casting the string to type <type>regtype</type>
(see <xref linkend="datatype-oid"/>), except that that will throw
error for name not found.
</para></entry>
</row>
<row>
<entry role="func_table_entry"><para role="func_signature">
<indexterm>
<primary>to_regtypemod</primary>
</indexterm>
<function>to_regtypemod</function> ( <type>text</type> )
<returnvalue>integer</returnvalue>
</para>
<para>
Parses a string of text, extracts a potential type name from it,
and translates its type modifier, if any. A syntax error in the
string will result in an error; but if the string is a
syntactically valid type name that happens not to be found in the
catalogs, the result is <literal>NULL</literal>. The result is
<literal>-1</literal> if no type modifier is present.
</para>
<para>
<function>to_regtypemod</function> can be combined with
<xref linkend="to-regtype"/> to produce appropriate inputs for
<xref linkend="format-type"/>, allowing a string representing a
type name to be canonicalized.
</para>
<para>
<literal>format_type(to_regtype('varchar(32)'), to_regtypemod('varchar(32)'))</literal>
<returnvalue>character varying(32)</returnvalue>
</para></entry>
</row>
</tbody>