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add documentation for tidb_enable_amend_pessimistic_txn. #4979

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Dec 10, 2020
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add documentation for tidb_enable_amend_pessimistic_txn. #4979

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537f5d0
Update system-variables.md
cfzjywxk Nov 30, 2020
8df077f
Update system-variables.md
cfzjywxk Dec 2, 2020
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Merge branch 'master' into system-variable-master-amend
TomShawn Dec 3, 2020
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Merge branch 'master' into system-variable-master-amend
TomShawn Dec 7, 2020
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Update system-variables.md
cfzjywxk Dec 10, 2020
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Merge branch 'master' into system-variable-master-amend
TomShawn Dec 10, 2020
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14 changes: 13 additions & 1 deletion system-variables.md
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Expand Up @@ -309,6 +309,18 @@ SET GLOBAL tidb_distsql_scan_concurrency = 10;
- 这个变量的值大于 `0` 时，TiDB 会将 `INSERT` 或 `LOAD DATA` 等语句在更小的事务中批量提交。这样可减少内存使用，确保大批量修改时事务大小不会达到 `txn-total-size-limit` 限制。
- 只有变量值为 `0` 时才符合 ACID 要求。否则无法保证 TiDB 的原子性和隔离性要求。

### `tidb_enable_amend_pessimistic_txn` <span class="version-mark">从 v4.0.7 版本开始引入</span>

- 作用域：SESSION | GLOBAL
- 默认值: 0
- 这个变量用于控制是否开启 `AMEND TRANSACTION` 特性。在[悲观事务模式](/pessimistic-transaction.md)下开启该特性后，如果该事务相关的表存在并发 DDL 操作和 SCHEMA VERSION 变更，TiDB 会尝试对该事务进行 amend 操作，修正该事务的提交内容，使其和最新的有效 SCHEMA VERSION 保持一致，从而成功提交该事务而不返回 `Information schema is changed` 报错。该特性对以下并发 DDL 变更生效：

- `ADD COLUMN` 或 `DROP COLUMN` 类型的 DDL 操作。
- `MODIFY COLUMN` 或 `CHANGE COLUMN` 类型的 DDL 操作，且只对增大字段长度的操作生效。
- `ADD INDEX` 或 `DROP INDEX` 类型的 DDL 操作，且操作的索引列须在事务开启之前创建。

目前该特性可能造成事务语义的变化，且与 TiDB Binlog 存在部分不兼容的场景，可以参考[事务语义行为区别](https://github.com/pingcap/tidb/issues/21069)和[与 TiDB Binlog 兼容问题汇总](https://github.com/pingcap/tidb/issues/20996)了解更多关于该特性的使用注意事项。

### `tidb_enable_cascades_planner`

- 作用域：SESSION | GLOBAL
Expand Down Expand Up @@ -972,4 +984,4 @@ explain select * from t where age=5;
- 作用域：SESSION
- 默认值：0.8
- TiDB 内存使用占总内存的比例超过一定阈值时会报警。该功能的详细介绍和使用方法可以参考 [`memory-usage-alarm-ratio`](/tidb-configuration-file.md#memory-usage-alarm-ratio)。
- 该变量的初始值可通过 [`memory-usage-alarm-ratio`](/tidb-configuration-file.md#memory-usage-alarm-ratio) 进行配置。
- 该变量的初始值可通过 [`memory-usage-alarm-ratio`](/tidb-configuration-file.md#memory-usage-alarm-ratio) 进行配置。