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Commit02c4ad3

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Doc: explicitly point out that enum values can't be dropped.
This was not stated in so many words anywhere. Document it to makeclear that it's a design limitation and not just an oversight ordocumentation omission.Discussion:https://postgr.es/m/152089733343.1222.6927268289645380498@wrigleys.postgresql.org
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‎doc/src/sgml/datatype.sgml

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@@ -3077,19 +3077,27 @@ SELECT person.name, holidays.num_weeks FROM person, holidays
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<sect2>
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<title>Implementation Details</title>
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<para>
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Enum labels are case sensitive, so
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<type>'happy'</type> is not the same as <type>'HAPPY'</type>.
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White space in the labels is significant too.
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</para>
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<para>
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Although enum types are primarily intended for static sets of values,
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there is support for adding new values to an existing enum type, and for
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renaming values (see <xref linkend="sql-altertype">). Existing values
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cannot be removed from an enum type, nor can the sort ordering of such
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values be changed, short of dropping and re-creating the enum type.
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</para>
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<para>
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An enum value occupies four bytes on disk. The length of an enum
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value's textual label is limited by the <symbol>NAMEDATALEN</symbol>
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setting compiled into <productname>PostgreSQL</productname>; in standard
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builds this means at most 63 bytes.
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</para>
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<para>
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Enum labels are case sensitive, so
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<type>'happy'</type> is not the same as <type>'HAPPY'</type>.
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White space in the labels is significant too.
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</para>
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<para>
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The translations from internal enum values to textual labels are
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kept in the system catalog

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