Doc: explicitly point out that enum values can't be dropped.

This was not stated in so many words anywhere.  Document it to make
clear that it's a design limitation and not just an oversight or
documentation omission.

Discussion: https://postgr.es/m/152089733343.1222.6927268289645380498@wrigleys.postgresql.org

1 files changed, 14 insertions, 6 deletions

diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index 9aa9b28f3e2..1736478a42b 100644
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -3060,6 +3060,20 @@ SELECT person.name, holidays.num_weeks FROM person, holidays
     <title>Implementation Details</title>
 
     <para>
+     Enum labels are case sensitive, so
+     <type>'happy'</type> is not the same as <type>'HAPPY'</type>.
+     White space in the labels is significant too.
+    </para>
+
+    <para>
+     Although enum types are primarily intended for static sets of values,
+     there is support for adding new values to an existing enum type, and for
+     renaming values (see <xref linkend="sql-altertype"/>).  Existing values
+     cannot be removed from an enum type, nor can the sort ordering of such
+     values be changed, short of dropping and re-creating the enum type.
+    </para>
+
+    <para>
      An enum value occupies four bytes on disk.  The length of an enum
      value's textual label is limited by the <symbol>NAMEDATALEN</symbol>
      setting compiled into <productname>PostgreSQL</productname>; in standard
@@ -3067,12 +3081,6 @@ SELECT person.name, holidays.num_weeks FROM person, holidays
     </para>
 
     <para>
-     Enum labels are case sensitive, so
-     <type>'happy'</type> is not the same as <type>'HAPPY'</type>.
-     White space in the labels is significant too.
-    </para>
-
-    <para>
      The translations from internal enum values to textual labels are
      kept in the system catalog
      <link linkend="catalog-pg-enum"><structname>pg_enum</structname></link>.