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Commitaf3ee8a

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Add some notes about why "ALTER TYPE enum DROP VALUE" is hard.
In hopes of putting these where any would-be implementer is sure tofind them, make a placeholder grammar production for ALTER DROP VALUEand put them there. This is really just a docs patch, though.Vik Fearing, with a bit more wordsmithing by meDiscussion:https://postgr.es/m/9fffd149-da0f-0c9c-6745-731fb688642a@postgresfriends.org
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‎src/backend/parser/gram.y

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@@ -6412,6 +6412,33 @@ AlterEnumStmt:
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n->skipIfNewValExists =false;
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$$ = (Node *) n;
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}
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|ALTERTYPE_Pany_nameDROPVALUE_PSconst
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{
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/*
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* The following problems must be solved before this can be
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* implemented:
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*
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* - There must be no instance of the target value in
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* any table.
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*
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* - The value must not appear in any catalog metadata,
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* such as stored view expressions or column defaults.
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*
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* - The value must not appear in any non-leaf page of a
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* btree (and similar issues with other index types).
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* This is problematic because a value could persist
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* there long after it's gone from user-visible data.
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*
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* - Concurrent sessions must not be able to insert the
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* value while the preceding conditions are being checked.
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*
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* - Possibly more...
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*/
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ereport(ERROR,
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(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
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errmsg("dropping an enum value is not implemented"),
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parser_errposition(@4)));
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}
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;
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opt_if_not_exists:IF_PNOTEXISTS {$$ =true; }

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