使用覆盖索引优化查询 [英] Optimization of query using covering indices
问题描述
我有以下带有子查询和自联接的查询:
I have the following query with a subquery and self join:
SELECT bucket.patient_sid AS sid
FROM
(SELECT clinical_data.patient_sid,
clinical_data.lft,
clinical_data.rgt
FROM clinical_data INNER JOIN
(SELECT clinical_data.patient_sid,
clinical_data.lft,
clinical_data.rgt,
clinical_data.attribute_id
FROM clinical_data
WHERE clinical_data.attribute_id = '33' AND clinical_data.string_value = '2160-0') AS attribute
ON clinical_data.patient_sid = attribute.patient_sid
AND clinical_data.lft >= attribute.lft
AND clinical_data.rgt <= attribute.rgt
WHERE clinical_data.attribute_id = '36') AS bucket;
我为此定义了以下索引:
I have the following indices defined on this:
KEY `idx_bucket` (`attribute_id`,`string_value`)
KEY `idx_self_join` (`patient_sid`,`attribute_id`,`lft`,`rgt`)
当我使用EXPLAIN查看查询时,使用覆盖索引idx_bucket的子查询肯定已优化,但是自联接和where子句却没有优化.此外,为什么它报告仅将patient_sid
和attribute_id
用于used_key_parts
,而将attachment_condition
显示为lft
,rgt
(这是什么意思?). lft
和'rgt`都被定义为没有特殊属性的整数,那么为什么不在我的索引中使用它们呢?
When I look at the query using EXPLAIN, the subquery using the covering index idx_bucket is definitely optimized, but the self join and where clause are not. Furthermore, why does it report that only patient_sid
and attribute_id
are used for used_key_parts
while an attachment_condition
is shown for lft
, rgt
(what does this mean?). Both lft
and 'rgt` are just defined as integers with no special properties, so why aren't they being used in my covering index?
当我定义时更奇怪
KEY `idx_self_join` (`patient_sid`,`lft`,`rgt`,`attribute_id`)
只有patient_sid
注册在used_key_parts.
中,而且filtered
从11.00%
跌至1.60%
!
only patient_sid
is registered in used_key_parts.
Furthermore filtered
drops to 1.60%
from 11.00%
!
{
"query_block": {
"select_id": 1,
"cost_info": {
"query_cost": "645186.71"
},
"nested_loop": [
{
"table": {
"table_name": "clinical_data",
"access_type": "ref",
"possible_keys": [
"fk_attribute_idx",
"idx_value_string",
"idx_value_double",
"idx_bucket",
"idx_self_join_idx"
],
"key": "idx_bucket",
"used_key_parts": [
"attribute_id",
"string_value"
],
"key_length": "308",
"ref": [
"const",
"const"
],
"rows_examined_per_scan": 126402,
"rows_produced_per_join": 126402,
"filtered": "100.00",
"cost_info": {
"read_cost": "126402.00",
"eval_cost": "25280.40",
"prefix_cost": "151682.40",
"data_read_per_join": "46M"
},
"used_columns": [
"patient_sid",
"string_value",
"attribute_id",
"lft",
"rgt"
],
"attached_condition": "(`ns_large2`.`clinical_data`.`patient_sid` is not null)"
}
},
{
"table": {
"table_name": "clinical_data",
"access_type": "ref",
"possible_keys": [
"fk_attribute_idx",
"idx_value_string",
"idx_value_double",
"idx_bucket",
"idx_self_join_idx"
],
"key": "idx_self_join_idx",
"used_key_parts": [
"attribute_id",
"patient_sid"
],
"key_length": "10",
"ref": [
"const",
"ns_large2.clinical_data.patient_sid"
],
"rows_examined_per_scan": 14,
"rows_produced_per_join": 201169,
"filtered": "11.11",
"using_index": true,
"cost_info": {
"read_cost": "131327.39",
"eval_cost": "40233.83",
"prefix_cost": "645186.71",
"data_read_per_join": "73M"
},
"used_columns": [
"patient_sid",
"attribute_id",
"lft",
"rgt"
],
"attached_condition": "((`ns_large2`.`clinical_data`.`lft` >= `ns_large2`.`clinical_data`.`lft`) and (`ns_large2`.`clinical_data`.`rgt` <= `ns_large2`.`clinical_data`.`rgt`))"
}
}
]
}
}
推荐答案
这是您的基本JOIN:
Here's your basic JOIN:
SELECT
FROM clinical_data cd1
JOIN clinical_data cd2
ON cd1.patient_sid = cd2.patient_sid
AND cd2.attribute_id = '33'
WHERE cd1.attribute_id = '36'
这篇关于使用覆盖索引优化查询的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!