From 3c28da8047ae4a673f37677d91df1fe284a53201 Mon Sep 17 00:00:00 2001 From: HE HUA Date: Thu, 17 Oct 2024 00:40:22 +0800 Subject: [PATCH] Correct deep-pagination-optimization.md In the section on deep pagination optimization recommendations, even with deferred joins and subqueries, the internal SQL statements will still result in deep pagination. These queries need to be rewritten to use the primary key index (id). Otherwise, there is no difference from the original SQL, and performance might even be worse. --- docs/high-performance/deep-pagination-optimization.md | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/docs/high-performance/deep-pagination-optimization.md b/docs/high-performance/deep-pagination-optimization.md index 8fa69f4a..0d39e627 100644 --- a/docs/high-performance/deep-pagination-optimization.md +++ b/docs/high-performance/deep-pagination-optimization.md @@ -64,12 +64,12 @@ SELECT * FROM t_order WHERE id > 100000 LIMIT 10 ```sql # 通过子查询来获取 id 的起始值，把 limit 1000000 的条件转移到子查询 -SELECT * FROM t_order WHERE id >= (SELECT id FROM t_order limit 1000000, 1) LIMIT 10; +SELECT * FROM t_order WHERE id >= (SELECT id FROM t_order where id > 1000000 limit 1) LIMIT 10; ``` **工作原理**: -1. 子查询 `(SELECT id FROM t_order LIMIT 1000000, 1)` 会利用主键索引快速定位到第 1000001 条记录，并返回其 ID 值。 +1. 子查询 `(SELECT id FROM t_order where id > 1000000 limit 1)` 会利用主键索引快速定位到第 1000001 条记录，并返回其 ID 值。 2. 主查询 `SELECT * FROM t_order WHERE id >= ... LIMIT 10` 将子查询返回的起始 ID 作为过滤条件，使用 `id >=` 获取从该 ID 开始的后续 10 条记录。 不过，子查询的结果会产生一张新表，会影响性能，应该尽量避免大量使用子查询。并且，这种方法只适用于 ID 是正序的。在复杂分页场景，往往需要通过过滤条件，筛选到符合条件的 ID，此时的 ID 是离散且不连续的。 @@ -84,12 +84,12 @@ SELECT * FROM t_order WHERE id >= (SELECT id FROM t_order limit 1000000, 1) LIMI -- 使用 INNER JOIN 进行延迟关联 SELECT t1.* FROM t_order t1 -INNER JOIN (SELECT id FROM t_order LIMIT 1000000, 10) t2 ON t1.id = t2.id; +INNER JOIN (SELECT id FROM t_order where id > 1000000 LIMIT 10) t2 ON t1.id = t2.id; ``` **工作原理**: -1. 子查询 `(SELECT id FROM t_order LIMIT 1000000, 10)` 利用主键索引快速定位目标分页的 10 条记录的 ID。 +1. 子查询 `(SELECT id FROM t_order where id > 1000000 LIMIT 10)` 利用主键索引快速定位目标分页的 10 条记录的 ID。 2. 通过 `INNER JOIN` 将子查询结果与主表 `t_order` 关联，获取完整的记录数据。 除了使用 INNER JOIN 之外，还可以使用逗号连接子查询。 @@ -97,7 +97,7 @@ INNER JOIN (SELECT id FROM t_order LIMIT 1000000, 10) t2 ON t1.id = t2.id; ```sql -- 使用逗号进行延迟关联 SELECT t1.* FROM t_order t1, -(SELECT id FROM t_order limit 1000000, 10) t2 +(SELECT id FROM t_order where id > 1000000 LIMIT 10) t2 WHERE t1.id = t2.id; ```