当我删除一个列时，Oracle 会发生什么？
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What happens in Oracle when I drop a column?
提问by trinity
What are the various things that happen when I drop a column from a filled table. In all_tab_columns, does the column_id of other columns get reset?
当我从填充的表格中删除一列时会发生哪些不同的事情。在 all_tab_columns 中，是否重置了其他列的 column_id？
回答by Jonathan Leffler
The actions will include at least the following:
行动将至少包括以下内容：
- The data stored in that column is lost.
- Views referencing that column are invalidated - but (according to Gary- thanks!) they're not dropped; they stay invalid until revised to work with the modified schema.
- Stored procedures referencing that column are invalidated - same caveat.
- The column ID numbers of following columns will be reset.
- Permissions granted on the column will be removed.
- Indexes referencing that column will be dropped.
- 存储在该列中的数据丢失。
- 引用该列的视图无效 - 但是（根据Gary- 谢谢！）它们没有被删除；在修改为使用修改后的架构之前，它们将保持无效。
- 引用该列的存储过程无效 - 同样的警告。
- 以下列的列 ID 编号将被重置。
- 对该列授予的权限将被删除。
- 引用该列的索引将被删除。
回答by Connor
alter table drop column
更改表删除列
will actually visit each block and remove the column data - and that's an expensive thing to do for a large table.
实际上会访问每个块并删除列数据 - 这对于大表来说是一件昂贵的事情。
You might find it more prudent to issue:
您可能会发现发出以下命令更为谨慎：
alter table set unused
更改表设置未使用
which just flags the column as "gone" in the dictionary. If you then still need to reclaim that space, you can schedule an "alter table drop unused" at a quiet time
它只是在字典中将该列标记为“消失”。如果您仍然需要回收该空间，您可以在安静的时间安排“更改表删除未使用”
