SQL 性能调优
本文档介绍一些导致 SQL 语句变慢的常见原因以及调优 SQL 性能的技巧。
在开始之前
你可以使用 tiup demo
import 来准备数据:
tiup demo bookshop prepare --host 127.0.0.1 --port 4000 --books 1000000
或者 使用 TiDB Cloud 的 Import 功能 导入预先准备好的示例数据。
问题:全表扫描
导致 SQL 查询变慢的最常见原因是 SELECT
语句执行了全表扫描或使用了不正确的索引。
当 TiDB 根据非主键列或二级索引中的列,从一个大型表中检索少量行时,性能通常较差:
SELECT * FROM books WHERE title = 'Marian Yost';
+------------+-------------+-----------------------+---------------------+-------+--------+
| id | title | type | published_at | stock | price |
+------------+-------------+-----------------------+---------------------+-------+--------+
| 65670536 | Marian Yost | Arts | 1950-04-09 06:28:58 | 542 | 435.01 |
| 1164070689 | Marian Yost | Education & Reference | 1916-05-27 12:15:35 | 216 | 328.18 |
| 1414277591 | Marian Yost | Arts | 1932-06-15 09:18:14 | 303 | 496.52 |
| 2305318593 | Marian Yost | Arts | 2000-08-15 19:40:58 | 398 | 402.90 |
| 2638226326 | Marian Yost | Sports | 1952-04-02 12:40:37 | 191 | 174.64 |
+------------+-------------+-----------------------+---------------------+-------+--------+
5 rows in set
Time: 0.582s
要理解为何此查询较慢,可以使用 EXPLAIN
查看执行计划:
EXPLAIN SELECT * FROM books WHERE title = 'Marian Yost';
+---------------------+------------+-----------+---------------+-----------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------+------------+-----------+---------------+-----------------------------------------+
| TableReader_7 | 1.27 | root | | data:Selection_6 |
| └─Selection_6 | 1.27 | cop[tikv] | | eq(bookshop.books.title, "Marian Yost") |
| └─TableFullScan_5 | 1000000.00 | cop[tikv] | table:books | keep order:false |
+---------------------+------------+-----------+---------------+-----------------------------------------+
从执行计划中的 TableFullScan_5
可以看出,TiDB 对 books
表执行了全表扫描,并逐行检查 title
是否满足条件。TableFullScan_5
的 estRows
值为 1000000.00
,意味着优化器估算此全表扫描会扫描 100 万行数据。
关于 EXPLAIN
的更多用法信息,请参见 EXPLAIN
Walkthrough。
解决方案:使用二级索引
为了加快上述查询的速度,可以在 books.title
列上添加二级索引:
CREATE INDEX title_idx ON books (title);
查询的执行速度会明显提升:
SELECT * FROM books WHERE title = 'Marian Yost';
+------------+-------------+-----------------------+---------------------+-------+--------+
| id | title | type | published_at | stock | price |
+------------+-------------+-----------------------+---------------------+-------+--------+
| 1164070689 | Marian Yost | Education & Reference | 1916-05-27 12:15:35 | 216 | 328.18 |
| 1414277591 | Marian Yost | Arts | 1932-06-15 09:18:14 | 303 | 496.52 |
| 2305318593 | Marian Yost | Arts | 2000-08-15 19:40:58 | 398 | 402.90 |
| 2638226326 | Marian Yost | Sports | 1952-04-02 12:40:37 | 191 | 174.64 |
| 65670536 | Marian Yost | Arts | 1950-04-09 06:28:58 | 542 | 435.01 |
+------------+-------------+-----------------------+---------------------+-------+--------+
5 rows in set
Time: 0.007s
为了理解性能提升的原因,可以用 EXPLAIN
查看新的执行计划:
EXPLAIN SELECT * FROM books WHERE title = 'Marian Yost';
+---------------------------+---------+-----------+-------------------------------------+-------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------------+---------+-----------+-------------------------------------+-------------------------------------------------------+
| IndexLookUp_10 | 1.27 | root | | |
| ├─IndexRangeScan_8(Build) | 1.27 | cop[tikv] | table:books, index:title_idx(title) | range:["Marian Yost","Marian Yost"], keep order:false |
| └─TableRowIDScan_9(Probe) | 1.27 | cop[tikv] | table:books | keep order:false |
+---------------------------+---------+-----------+-------------------------------------+-------------------------------------------------------+
从执行计划中的 IndexLookUp_10
可以看出,TiDB 通过 title_idx
索引进行数据查询。其 estRows
为 1.27
,意味着优化器估算只会扫描大约 1.27 行数据。相比全表扫描的 100 万行,显著减少了扫描行数。
IndexLookup_10
的执行流程是:先通过 IndexRangeScan_8
操作读取满足条件的索引数据,然后通过存储在索引中的 Row ID,使用 TableLookup_9
操作查询对应的行。
关于 TiDB 执行计划的更多信息,请参见 TiDB Query Execution Plan Overview。
解决方案:使用覆盖索引
如果索引是覆盖索引,且包含所有 SQL 语句查询的列,则只扫描索引数据即可满足查询。
例如,以下查询只需要根据 title
查询对应的 price
:
SELECT title, price FROM books WHERE title = 'Marian Yost';
+-------------+--------+
| title | price |
+-------------+--------+
| Marian Yost | 435.01 |
| Marian Yost | 328.18 |
| Marian Yost | 496.52 |
| Marian Yost | 402.90 |
| Marian Yost | 174.64 |
+-------------+--------+
5 rows in set
Time: 0.007s
由于 title_idx
索引只包含 title
列的数据,TiDB 仍需先扫描索引数据,然后再从表中查询 price
列。
用 EXPLAIN
查看执行计划:
EXPLAIN SELECT title, price FROM books WHERE title = 'Marian Yost';
+---------------------------+---------+-----------+-------------------------------------+-------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------------+---------+-----------+-------------------------------------+-------------------------------------------------------+
| IndexLookUp_10 | 1.27 | root | | |
| ├─IndexRangeScan_8(Build) | 1.27 | cop[tikv] | table:books, index:title_idx(title) | range:["Marian Yost","Marian Yost"], keep order:false |
| └─TableRowIDScan_9(Probe) | 1.27 | cop[tikv] | table:books | keep order:false |
+---------------------------+---------+-----------+-------------------------------------+-------------------------------------------------------+
为了优化性能,可以删除旧的索引 title_idx
,并创建一个新的覆盖索引 title_price_idx
:
ALTER TABLE books DROP INDEX title_idx;
CREATE INDEX title_price_idx ON books (title, price);
因为 price
数据存储在 title_price_idx
索引中,以下查询只需扫描索引数据:
EXPLAIN SELECT title, price FROM books WHERE title = 'Marian Yost';
+---------------------+---------+-----------+--------------------------------------------------+-------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------+---------+-----------+--------------------------------------------------+-------------------------------------------------------+
| IndexReader_6 | 1.27 | root | | index:IndexRangeScan_5 |
| └─IndexRangeScan_5 | 1.27 | cop[tikv] | table:books, index:title_price_idx(title, price) | range:["Marian Yost","Marian Yost"], keep order:false |
+---------------------+---------+-----------+--------------------------------------------------+-------------------------------------------------------+
此时,查询运行速度更快:
SELECT title, price FROM books WHERE title = 'Marian Yost';
+-------------+--------+
| title | price |
+-------------+--------+
| Marian Yost | 174.64 |
| Marian Yost | 328.18 |
| Marian Yost | 402.90 |
| Marian Yost | 435.01 |
| Marian Yost | 496.52 |
+-------------+--------+
5 rows in set
Time: 0.004s
由于 books
表会在后续示例中使用,建议删除 title_price_idx
索引:
ALTER TABLE books DROP INDEX title_price_idx;
解决方案:使用主键索引
如果查询使用主键过滤数据,查询会非常快。例如,books
表的主键是 id
列,可以用 id
列进行查询:
SELECT * FROM books WHERE id = 896;
+-----+----------------+----------------------+---------------------+-------+--------+
| id | title | type | published_at | stock | price |
+-----+----------------+----------------------+---------------------+-------+--------+
| 896 | Kathryne Doyle | Science & Technology | 1969-03-18 01:34:15 | 468 | 281.32 |
+-----+----------------+----------------------+---------------------+-------+--------+
1 row in set
Time: 0.004s
用 EXPLAIN
查看执行计划:
EXPLAIN SELECT * FROM books WHERE id = 896;
+--------------+---------+------+---------------+---------------+
| id | estRows | task | access object | operator info |
+--------------+---------+------+---------------+---------------+
| Point_Get_1 | 1.00 | root | table:books | handle:896 |
+--------------+---------+------+---------------+---------------+
Point_Get
是一种非常快速的执行计划。
使用正确的连接类型
请参见 JOIN 执行计划。