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87 lines
2.5 KiB
Markdown
87 lines
2.5 KiB
Markdown
---
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slug: /zh/sql-reference/statements/alter/ttl
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sidebar_position: 44
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sidebar_label: TTL
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---
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# 表的 TTL 操作 {#manipulations-with-table-ttl}
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## 修改 MODIFY TTL {#modify-ttl}
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你能修改 [表 TTL](../../../engines/table-engines/mergetree-family/mergetree.md#mergetree-table-ttl) ，命令语法如下所示：
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``` sql
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ALTER TABLE table_name MODIFY TTL ttl_expression;
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```
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## 移除 REMOVE TTL {#remove-ttl}
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TTL 属性可以用下列命令从表中移除:
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```sql
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ALTER TABLE table_name REMOVE TTL
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```
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**示例**
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创建一个表，带有 `TTL` 属性如下所示:
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```sql
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CREATE TABLE table_with_ttl
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(
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event_time DateTime,
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UserID UInt64,
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Comment String
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)
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ENGINE MergeTree()
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ORDER BY tuple()
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TTL event_time + INTERVAL 3 MONTH;
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SETTINGS min_bytes_for_wide_part = 0;
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INSERT INTO table_with_ttl VALUES (now(), 1, 'username1');
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INSERT INTO table_with_ttl VALUES (now() - INTERVAL 4 MONTH, 2, 'username2');
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```
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运行命令 `OPTIMIZE` 强制清理 `TTL`:
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```sql
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OPTIMIZE TABLE table_with_ttl FINAL;
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SELECT * FROM table_with_ttl FORMAT PrettyCompact;
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```
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第二行记录被从表中删除掉了.
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```text
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┌─────────event_time────┬──UserID─┬─────Comment──┐
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│ 2020-12-11 12:44:57 │ 1 │ username1 │
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└───────────────────────┴─────────┴──────────────┘
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```
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现在用下面的命令，把表的 `TTL` 移除掉:
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```sql
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ALTER TABLE table_with_ttl REMOVE TTL;
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```
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重新插入上面的数据，并尝试再次运行 `OPTIMIZE` 命令清理 `TTL` 属性 :
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```sql
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INSERT INTO table_with_ttl VALUES (now() - INTERVAL 4 MONTH, 2, 'username2');
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OPTIMIZE TABLE table_with_ttl FINAL;
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SELECT * FROM table_with_ttl FORMAT PrettyCompact;
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```
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可以看得到 `TTL` 这个属性已经没了，并且可以看得到第二行记录并没有被删除:
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```text
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┌─────────event_time────┬──UserID─┬─────Comment──┐
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│ 2020-12-11 12:44:57 │ 1 │ username1 │
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│ 2020-08-11 12:44:57 │ 2 │ username2 │
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└───────────────────────┴─────────┴──────────────┘
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```
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**更多参考**
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- 关于 [TTL 表达式](../../../sql-reference/statements/create/table.mdx#ttl-expression).
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- 修改列 [with TTL](../../../sql-reference/statements/alter/column.mdx#alter_modify-column).
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