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alter table [TABLE_NAME] add temp varchar2(4000);
update [TABLE_NAME] set temp = [VARCHAR2_NAME];
update [TABLE_NAME] set [VARCHAR2_NAME] = null;
commit;

alter table [TABLE_NAME] modify [VARCHAR2_NAME] long;
alter table [TABLE_NAME] modify [VARCHAR2_NAME] clob;
update [TABLE_NAME] set [VARCHAR2_NAME] = temp;
alter table [TABLE_NAME] drop column temp;
commit;

//----某些情况下，是需要对已有索引更新
alter index [INDEX_NAME] rebuild;