SQL优化--一般步骤
使用MySQL案例库：sakila
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sakila |
| staffdb |
| sys |
| test |
| test1 |
| world |
+--------------------+
9 rows in set (0.00 sec)mysql> use sakila;
Database changed
mysql> show tables;
+----------------------------+
| Tables_in_sakila |
+----------------------------+
| actor |
| actor_info |
| address |
| category |
| city |
| country |
| customer |
| customer_list |
| film |
| film_actor |
| film_category |
| film_list |
| film_text |
| inventory |
| language |
| nicer_but_slower_film_list |
| payment |
| rental |
| sales_by_film_category |
| sales_by_store |
| staff |
| staff_list |
| store |
+----------------------------+
23 rows in set (0.03 sec)
1.定位执行效率低的SQL语句
- 慢查询日志，用--log-slow-queries[=file_name]选项启动时，mysqld写一个包含所有执行时间超过long_query_time秒的SQL语句的日志文件；
- 慢查询日志在查询结束后记录，在应用反映执行效率出现问题时，慢查询日志不能定位问题，可以使用show processlist命令查看当前MySQL 在进行的进程，包括线程的状态，是否表锁等，可以实时查看SQL执行情况，同时对一些锁表操作进行优化。
2.使用explain分析低效SQL的执行计划
mysql> explain select sum(amount)from customer a,payment b where a.customer_id=b.customer_id
-> and email='[email protected]'\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: a
partitions: NULL
type: ALL
possible_keys: PRIMARY
key: NULL
key_len: NULL
ref: NULL
rows: 599
filtered: 10.00
Extra: Using where
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: b
partitions: NULL
type: ref
possible_keys: idx_fk_customer_id
key: idx_fk_customer_id
key_len: 2
ref: sakila.a.customer_id
rows: 26
filtered: 100.00
Extra: NULL
2 rows in set, 1 warning (0.06 sec)使用explain extended 加上show warnings可以看到SQL在真正被执行之前优化器做了哪些SQL改写。
3.通过show profile 分析SQL
有时候仅仅通过explain分析执行计划不能很快定位。
查看当前MySQL是否支持profile：
mysql> select @@have_profiling;
+------------------+
| @@have_profiling |
+------------------+
| YES |
+------------------+
1 row in set, 1 warning (0.06 sec)默认profiling关闭，用set语句在session级别开启profiling：
mysql> select @@profiling;
+-------------+
| @@profiling |
+-------------+
| 0 |
+-------------+
1 row in set, 1 warning (0.03 sec)
mysql> set profiling = 1;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> select @@profiling;
+-------------+
| @@profiling |
+-------------+
| 1 |
+-------------+
1 row in set, 1 warning (0.03 sec)可以看到具体时间花费在哪里：
mysql> select count(*) from payment;
+----------+
| count(*) |
+----------+
| 16049 |
+----------+
1 row in set (0.08 sec)
mysql> show profiles;
+----------+------------+------------------------------+
| Query_ID | Duration | Query |
+----------+------------+------------------------------+
| 1 | 0.02803550 | select @@profiling |
| 2 | 0.08277525 | select count(*) from payment |
+----------+------------+------------------------------+
2 rows in set, 1 warning (0.03 sec)
mysql> show profile for query 2;
+----------------------+----------+
| Status | Duration |
+----------------------+----------+
| starting | 0.030659 |
| checking permissions | 0.000043 |
| Opening tables | 0.000042 |
| init | 0.000024 |
| System lock | 0.000017 |
| optimizing | 0.000011 |
| statistics | 0.000056 |
| preparing | 0.000019 |
| executing | 0.000003 |
| Sending data | 0.051475 |
| end | 0.000095 |
| query end | 0.000051 |
| closing tables | 0.000020 |
| freeing items | 0.000234 |
| cleaning up | 0.000027 |
+----------------------+----------+
15 rows in set, 1 warning (0.04 sec)4.通过trace分析优化器如何选择执行计划
能够进一步了解为什么优化器选择A执行计划而不选择B执行计划。、
打开trace:
mysql> set optimizer_trace="enabled=on";
Query OK, 0 rows affected (0.00 sec)设置trace可使用的最大内存大小：
mysql> SET OPTIMIZER_TRACE_MAX_MEM_SIZE=1000000;
Query OK, 0 rows affected (0.03 sec)MySQL是如何执行SQL：
mysql> select rental_id from rental where 1=1 and rental_date >= '2005-05-25 04:00:00'and rental_date <= '2005-05-25 05:00:00' and inventory_id=4466;
+-----------+
| rental_id |
+-----------+
| 39 |
+-----------+
1 row in set (0.00 sec)
mysql> select * from information_schema.optimizer_trace\G
*************************** 1. row ***************************
QUERY: select rental_id from rental where 1=1 and rental_date >= '2005-05-25 04:00:00'and rental_date <= '2005-05-25 05:00:00' and inventory_id=4466
TRACE: {
"steps": [
{
"join_preparation": {
"select#": 1,
"steps": [
{
"expanded_query": "/* select#1 */ select `rental`.`rental_id` AS `rental_id` from `rental` where ((1 = 1) and (`rental`.`rental_date` >= '2005-05-25 04:00:00') and (`rental`.`rental_date` <= '2005-05-25 05:00:00') and (`rental`.`inventory_id` = 4466))"
}
]
}
},
{
"join_optimization": {
"select#": 1,
"steps": [
{
"condition_processing": {
"condition": "WHERE",
"original_condition": "((1 = 1) and (`rental`.`rental_date` >= '2005-05-25 04:00:00') and (`rental`.`rental_date` <= '2005-05-25 05:00:00') and (`rental`.`inventory_id` = 4466))",
"steps": [
{
"transformation": "equality_propagation",
"resulting_condition": "((1 = 1) and (`rental`.`rental_date` >= '2005-05-25 04:00:00') and (`rental`.`rental_date` <= '2005-05-25 05:00:00') and multiple equal(4466, `rental`.`inventory_id`))"
},
{
"transformation": "constant_propagation",
"resulting_condition": "((1 = 1) and (`rental`.`rental_date` >= '2005-05-25 04:00:00') and (`rental`.`rental_date` <= '2005-05-25 05:00:00') and multiple equal(4466, `rental`.`inventory_id`))"
},
{
"transformation": "trivial_condition_removal",
"resulting_condition": "((`rental`.`rental_date` >= '2005-05-25 04:00:00') and (`rental`.`rental_date` <= '2005-05-25 05:00:00') and multiple equal(4466, `rental`.`inventory_id`))"
}
]
}
},
{
"substitute_generated_columns": {
}
},
{
"table_dependencies": [
{
"table": "`rental`",
"row_may_be_null": false,
"map_bit": 0,
"depends_on_map_bits": [
]
}
]
},
{
"ref_optimizer_key_uses": [
{
"table": "`rental`",
"field": "inventory_id",
"equals": "4466",
"null_rejecting": false
}
]
},
{
"rows_estimation": [
{
"table": "`rental`",
"range_analysis": {
"table_scan": {
"rows": 16008,
"cost": 3300.7
},
"potential_range_indexes": [
{
"index": "PRIMARY",
"usable": false,
"cause": "not_applicable"
},
{
"index": "rental_date",
"usable": true,
"key_parts": [
"rental_date",
"inventory_id",
"customer_id"
]
},
{
"index": "idx_fk_inventory_id",
"usable": true,
"key_parts": [
"inventory_id",
"rental_id"
]
},
{
"index": "idx_fk_customer_id",
"usable": false,
"cause": "not_applicable"
},
{
"index": "idx_fk_staff_id",
"usable": false,
"cause": "not_applicable"
}
],
"best_covering_index_scan": {
"index": "rental_date",
"cost": 3229.9,
"chosen": true
},
"setup_range_conditions": [
],
"group_index_range": {
"chosen": false,
"cause": "not_group_by_or_distinct"
},
"analyzing_range_alternatives": {
"range_scan_alternatives": [
{
"index": "rental_date",
"ranges": [
"0x9975b24000 <= rental_date <= 0x9975b25000"
],
"index_dives_for_eq_ranges": true,
"rowid_ordered": false,
"using_mrr": false,
"index_only": true,
"rows": 10,
"cost": 3.0254,
"chosen": true
},
{
"index": "idx_fk_inventory_id",
"ranges": [
"4466 <= inventory_id <= 4466"
],
"index_dives_for_eq_ranges": true,
"rowid_ordered": true,
"using_mrr": false,
"index_only": false,
"rows": 5,
"cost": 7.01,
"chosen": false,
"cause": "cost"
}
],
"analyzing_roworder_intersect": {
"usable": false,
"cause": "too_few_roworder_scans"
}
},
"chosen_range_access_summary": {
"range_access_plan": {
"type": "range_scan",
"index": "rental_date",
"rows": 10,
"ranges": [
"0x9975b24000 <= rental_date <= 0x9975b25000"
]
},
"rows_for_plan": 10,
"cost_for_plan": 3.0254,
"chosen": true
}
}
}
]
},
{
"considered_execution_plans": [
{
"plan_prefix": [
],
"table": "`rental`",
"best_access_path": {
"considered_access_paths": [
{
"access_type": "ref",
"index": "idx_fk_inventory_id",
"rows": 5,
"cost": 6,
"chosen": true
},
{
"rows_to_scan": 10,
"access_type": "range",
"range_details": {
"used_index": "rental_date"
},
"resulting_rows": 10,
"cost": 5.0254,
"chosen": true
}
]
},
"condition_filtering_pct": 100,
"rows_for_plan": 10,
"cost_for_plan": 5.0254,
"chosen": true
}
]
},
{
"attaching_conditions_to_tables": {
"original_condition": "((`rental`.`inventory_id` = 4466) and (`rental`.`rental_date` >= '2005-05-25 04:00:00') and (`rental`.`rental_date` <= '2005-05-25 05:00:00'))",
"attached_conditions_computation": [
],
"attached_conditions_summary": [
{
"table": "`rental`",
"attached": "((`rental`.`inventory_id` = 4466) and (`rental`.`rental_date` >= '2005-05-25 04:00:00') and (`rental`.`rental_date` <= '2005-05-25 05:00:00'))"
}
]
}
},
{
"refine_plan": [
{
"table": "`rental`"
}
]
}
]
}
},
{
"join_execution": {
"select#": 1,
"steps": [
]
}
}
]
}
MISSING_BYTES_BEYOND_MAX_MEM_SIZE: 0
INSUFFICIENT_PRIVILEGES: 0
1 row in set (0.00 sec)5.确定问题并采取相应措施
加索引，以及常用的优化方法。。
见下篇。