Doc: improve and centralize the documentation for OID alias types.

Previously, a lot of information about type regclass existed only
in the discussion of the sequence functions.  Maybe that made sense
in the beginning, because I think originally those were the only
functions taking regclass.  But it doesn't make sense anymore.
Move that material to the "Object Identifier Types" section in
datatype.sgml, generalize it to talk about the other reg* types
as well, and add more examples.

Per bug #16991 from Federico Caselli.

Discussion: https://postgr.es/m/16991-bcaeaafa17e0a723@postgresql.org

doc/src/sgml/datatype.sgml

@@ -4632,7 +4632,8 @@ INSERT INTO mytable VALUES(-1);  -- fails
     <productname>PostgreSQL</productname> as primary keys for various
     system tables.
     Type <type>oid</type> represents an object identifier.  There are also
-    several alias types for <type>oid</type> named <type>reg<replaceable>something</replaceable></type>.
+    several alias types for <type>oid</type>, each
+    named <type>reg<replaceable>something</replaceable></type>.
     <xref linkend="datatype-oid-table"/> shows an
     overview.
    </para>
@@ -4780,10 +4781,14 @@ SELECT * FROM pg_attribute
     </table>
 
    <para>
-    All of the OID alias types for objects grouped by namespace accept
-    schema-qualified names, and will
+    All of the OID alias types for objects that are grouped by namespace
+    accept schema-qualified names, and will
     display schema-qualified names on output if the object would not
     be found in the current search path without being qualified.
+    For example, <literal>myschema.mytable</literal> is acceptable input
+    for <type>regclass</type> (if there is such a table).  That value
+    might be output as <literal>myschema.mytable</literal>, or
+    just <literal>mytable</literal>, depending on the current search path.
     The <type>regproc</type> and <type>regoper</type> alias types will only
     accept input names that are unique (not overloaded), so they are
     of limited use; for most uses <type>regprocedure</type> or
@@ -4792,6 +4797,87 @@ SELECT * FROM pg_attribute
     operand.
    </para>
 
+   <para>
+    The input functions for these types allow whitespace between tokens,
+    and will fold upper-case letters to lower case, except within double
+    quotes; this is done to make the syntax rules similar to the way
+    object names are written in SQL.  Conversely, the output functions
+    will use double quotes if needed to make the output be a valid SQL
+    identifier.  For example, the OID of a function
+    named <literal>Foo</literal> (with upper case <literal>F</literal>)
+    taking two integer arguments could be entered as
+    <literal>' "Foo" ( int, integer ) '::regprocedure</literal>.  The
+    output would look like <literal>"Foo"(integer,integer)</literal>.
+    Both the function name and the argument type names could be
+    schema-qualified, too.
+   </para>
+
+   <para>
+    Many built-in <productname>PostgreSQL</productname> functions accept
+    the OID of a table, or another kind of database object, and for
+    convenience are declared as taking <type>regclass</type> (or the
+    appropriate OID alias type).  This means you do not have to look up
+    the object's OID by hand, but can just enter its name as a string
+    literal.  For example, the <function>nextval(regclass)</function> function
+    takes a sequence relation's OID, so you could call it like this:
+<programlisting>
+nextval('foo')              <lineannotation>operates on sequence <literal>foo</literal></lineannotation>
+nextval('FOO')              <lineannotation>same as above</lineannotation>
+nextval('"Foo"')            <lineannotation>operates on sequence <literal>Foo</literal></lineannotation>
+nextval('myschema.foo')     <lineannotation>operates on <literal>myschema.foo</literal></lineannotation>
+nextval('"myschema".foo')   <lineannotation>same as above</lineannotation>
+nextval('foo')              <lineannotation>searches search path for <literal>foo</literal></lineannotation>
+</programlisting>
+   </para>
+
+   <note>
+    <para>
+     When you write the argument of such a function as an unadorned
+     literal string, it becomes a constant of type <type>regclass</type>
+     (or the appropriate type).
+     Since this is really just an OID, it will track the originally
+     identified object despite later renaming, schema reassignment,
+     etc.  This <quote>early binding</quote> behavior is usually desirable for
+     object references in column defaults and views.  But sometimes you might
+     want <quote>late binding</quote> where the object reference is resolved
+     at run time.  To get late-binding behavior, force the constant to be
+     stored as a <type>text</type> constant instead of <type>regclass</type>:
+<programlisting>
+nextval('foo'::text)      <lineannotation><literal>foo</literal> is looked up at runtime</lineannotation>
+</programlisting>
+     The <function>to_regclass()</function> function and its siblings
+     can also be used to perform run-time lookups.  See
+     <xref linkend="functions-info-catalog-table"/>.
+    </para>
+   </note>
+
+   <para>
+    Another practical example of use of <type>regclass</type>
+    is to look up the OID of a table listed in
+    the <literal>information_schema</literal> views, which don't supply
+    such OIDs directly.  One might for example wish to call
+    the <function>pg_relation_size()</function> function, which requires
+    the table OID.  Taking the above rules into account, the correct way
+    to do that is
+<programlisting>
+SELECT table_schema, table_name,
+       pg_relation_size((quote_ident(table_schema) || '.' ||
+                         quote_ident(table_name))::regclass)
+FROM information_schema.tables
+WHERE ...
+</programlisting>
+    The <function>quote_ident()</function> function will take care of
+    double-quoting the identifiers where needed.  The seemingly easier
+<programlisting>
+SELECT pg_relation_size(table_name)
+FROM information_schema.tables
+WHERE ...
+</programlisting>
+    is <emphasis>not recommended</emphasis>, because it will fail for
+    tables that are outside your search path or have names that require
+    quoting.
+   </para>
+
    <para>
     An additional property of most of the OID alias types is the creation of
     dependencies.  If a
@@ -4801,19 +4887,13 @@ SELECT * FROM pg_attribute
     expression <literal>nextval('my_seq'::regclass)</literal>,
     <productname>PostgreSQL</productname>
     understands that the default expression depends on the sequence
-    <literal>my_seq</literal>; the system will not let the sequence be dropped
-    without first removing the default expression.
-    <type>regrole</type> is the only exception for the property. Constants of this
-    type are not allowed in such expressions.
-   </para>
-
-   <note>
-   <para>
-    The OID alias types do not completely follow transaction isolation
-    rules. The planner also treats them as simple constants, which may
-    result in sub-optimal planning.
+    <literal>my_seq</literal>, so the system will not let the sequence
+    be dropped without first removing the default expression.  The
+    alternative of <literal>nextval('my_seq'::text)</literal> does not
+    create a dependency.
+    (<type>regrole</type> is an exception to this property. Constants of this
+    type are not allowed in stored expressions.)
    </para>
-   </note>
 
    <para>
     Another identifier type used by the system is <type>xid</type>, or transaction

doc/src/sgml/func.sgml

@@ -14429,8 +14429,9 @@ SELECT xmltable.*
     <function>table_to_xml</function> maps the content of the named
     table, passed as parameter <parameter>table</parameter>.  The
     <type>regclass</type> type accepts strings identifying tables using the
-    usual notation, including optional schema qualifications and
-    double quotes.  <function>query_to_xml</function> executes the
+    usual notation, including optional schema qualification and
+    double quotes (see <xref linkend="datatype-oid"/> for details).
+    <function>query_to_xml</function> executes the
     query whose text is passed as parameter
     <parameter>query</parameter> and maps the result set.
     <function>cursor_to_xml</function> fetches the indicated number of
@@ -17316,49 +17317,9 @@ SELECT setval('myseq', 42, false);    <lineannotation>Next <function>nextval</fu
    a <type>regclass</type> argument, which is simply the OID of the sequence in the
    <structname>pg_class</structname> system catalog.  You do not have to look up the
    OID by hand, however, since the <type>regclass</type> data type's input
-   converter will do the work for you.  Just write the sequence name enclosed
-   in single quotes so that it looks like a literal constant.  For
-   compatibility with the handling of ordinary
-   <acronym>SQL</acronym> names, the string will be converted to lower case
-   unless it contains double quotes around the sequence name.  Thus:
-<programlisting>
-nextval('foo')      <lineannotation>operates on sequence <literal>foo</literal></lineannotation>
-nextval('FOO')      <lineannotation>operates on sequence <literal>foo</literal></lineannotation>
-nextval('"Foo"')    <lineannotation>operates on sequence <literal>Foo</literal></lineannotation>
-</programlisting>
-   The sequence name can be schema-qualified if necessary:
-<programlisting>
-nextval('myschema.foo')     <lineannotation>operates on <literal>myschema.foo</literal></lineannotation>
-nextval('"myschema".foo')   <lineannotation>same as above</lineannotation>
-nextval('foo')              <lineannotation>searches search path for <literal>foo</literal></lineannotation>
-</programlisting>
-   See <xref linkend="datatype-oid"/> for more information about
-   <type>regclass</type>.
+   converter will do the work for you.  See <xref linkend="datatype-oid"/>
+   for details.
   </para>
-
-  <note>
-   <para>
-    When you write the argument of a sequence function as an unadorned
-    literal string, it becomes a constant of type <type>regclass</type>.
-    Since this is really just an OID, it will track the originally
-    identified sequence despite later renaming, schema reassignment,
-    etc.  This <quote>early binding</quote> behavior is usually desirable for
-    sequence references in column defaults and views.  But sometimes you might
-    want <quote>late binding</quote> where the sequence reference is resolved
-    at run time.  To get late-binding behavior, force the constant to be
-    stored as a <type>text</type> constant instead of <type>regclass</type>:
-<programlisting>
-nextval('foo'::text)      <lineannotation><literal>foo</literal> is looked up at runtime</lineannotation>
-</programlisting>
-   </para>
-
-   <para>
-    Of course, the argument of a sequence function can be an expression
-    as well as a constant.  If it is a text expression then the implicit
-    coercion will result in a run-time lookup.
-   </para>
-  </note>
-
  </sect1>
 
 
@@ -26474,11 +26435,8 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
     <type>regclass</type> argument, which is simply the OID of the table or index
     in the <structname>pg_class</structname> system catalog.  You do not have to look up
     the OID by hand, however, since the <type>regclass</type> data type's input
-    converter will do the work for you.  Just write the table name enclosed in
-    single quotes so that it looks like a literal constant.  For compatibility
-    with the handling of ordinary <acronym>SQL</acronym> names, the string
-    will be converted to lower case unless it contains double quotes around
-    the table name.
+    converter will do the work for you.  See <xref linkend="datatype-oid"/>
+    for details.
    </para>
 
    <para>