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new 4b11e3f100c Fix rewrite document (#23590)
4b11e3f100c is described below
commit 4b11e3f100c9c2ba0753d072f0717e42cc4266c8
Author: ZhangCheng <flyin...@outlook.com>
AuthorDate: Tue Jan 17 08:11:18 2023 +0800
Fix rewrite document (#23590)
---
docs/document/content/reference/sharding/rewrite.cn.md | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/docs/document/content/reference/sharding/rewrite.cn.md
b/docs/document/content/reference/sharding/rewrite.cn.md
index 1e98e526b0d..1ab9755e213 100644
--- a/docs/document/content/reference/sharding/rewrite.cn.md
+++ b/docs/document/content/reference/sharding/rewrite.cn.md
@@ -126,7 +126,7 @@ SELECT o.* FROM t_order o, t_order_item i WHERE
o.order_id=i.order_id ORDER BY u
SELECT o.*, order_item_id AS ORDER_BY_DERIVED_0 FROM t_order o, t_order_item i
WHERE o.order_id=i.order_id ORDER BY user_id, order_item_id;
```
-补列的另一种情况是使用 AVG 聚合函数。在分布式的场景中，使用 avg1 + avg2 + avg3 / 3 计算平均值并不正确，需要改写为 (sum1
+ sum2 + sum3) / (count1 + count2 + count3)。
+补列的另一种情况是使用 AVG 聚合函数。在分布式的场景中，使用 (avg1 + avg2 + avg3) / 3 计算平均值并不正确，需要改写为
(sum1 + sum2 + sum3) / (count1 + count2 + count3)。
这就需要将包含 AVG 的 SQL 改写为 SUM 和 COUNT，并在结果归并时重新计算平均值。例如以下 SQL：
```sql
@@ -207,15 +207,15 @@ SELECT score FROM t_score ORDER BY score DESC LIMIT 1, 2;
举例说明，如下 SQL：
```sql
-INSERT INTO t_order (order_id, xxx) VALUES (1, 'xxx'), (2, 'xxx'), (3, 'xxx');
+INSERT INTO t_order (order_id, xxx) VALUES (1, 'xxx'), (2, 'xxx'), (3, 'xxx');
```
假设数据库仍然是按照 order_id 的奇偶值分为两片的，仅将这条 SQL 中的表名进行修改，然后发送至数据库完成 SQL 的执行
，则两个分片都会写入相同的记录。
虽然只有符合分片查询条件的数据才能够被查询语句取出，但存在冗余数据的实现方案并不合理。因此需要将 SQL 改写为：
```sql
-INSERT INTO t_order_0 (order_id, xxx) VALUES (2, 'xxx');
-INSERT INTO t_order_1 (order_id, xxx) VALUES (1, 'xxx'), (3, 'xxx');
+INSERT INTO t_order_0 (order_id, xxx) VALUES (2, 'xxx');
+INSERT INTO t_order_1 (order_id, xxx) VALUES (1, 'xxx'), (3, 'xxx');
```
使用 IN 的查询与批量插入的情况相似，不过 IN 操作并不会导致数据查询结果错误。通过对 IN 查询的改写，可以进一步的提升查询性能。如以下 SQL：
