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Different "Fetch Time" for Same Query with Diff Indexes
Posted by: Deepak Gupta
Date: March 18, 2018 04:04PM

Hi There

I an using MySQL 5.7.20 with my schema on InnoDB engine.
My schema four tables:
1. "Collections_history" with approx 60 Million Records
2. "Attribute" with 6 records
3. "Attribute_grp" with 6 records
4. "Audit_Grp" with 6 records.

The "collections_history" table contains time series data that apart from timestamp and an integer measured value contains three foreign keys referring back to rest of the three tables.


Now I want to run the following query in the most efficient way:

select t_attribute.name, t_attribute_grp.name, t_audit_grp.name, metric_value from collections_history_table join t_attribute ON t_attribute.id = fkid_attribute join t_attribute_grp ON t_attribute_grp.id = fkid_attribute_grp_name join t_audit_grp ON t_audit_grp.id = fkid_audit_grp_name where t_attribute.name like "%CPU Usage%";

Here is the "Create" query for the collections_history table:
CREATE TABLE `collections_history_table` (
`idcollections_history_table` int(11) NOT NULL AUTO_INCREMENT,
`metric_value` int(11) NOT NULL,
`date_insert` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
`fkid_audit_grp_name` int(11) NOT NULL,
`fkid_attribute_grp_name` int(11) NOT NULL,
`fkid_attribute` int(11) NOT NULL,
PRIMARY KEY (`idcollections_history_table`),
UNIQUE KEY `idcollections_history_table_UNIQUE` (`idcollections_history_table`),
) ENGINE=InnoDB AUTO_INCREMENT=141352835 DEFAULT CHARSET=latin1;


On my 2.2 Ghz Quad core machine with ~approx 7G given to MySQL instance I get a followings stats:
- "Query Duration" ranges from 0.003 to 0.03 Secs
- "Query Fetch Time" ranges from 35 to 60 Secs


In my attempt to optimize performance I added three additional indexes to the "collections_history" table:

KEY `index3` (`fkid_audit_grp_name`),
KEY `index4` (`fkid_attribute_grp_name`),
KEY `index5` (`fkid_attribute`)

The results post the change I got were as follows:
- "Query Duration" ranges from 0.005 to 0.02 Secs
- "Query Fetch Time" ranges from 740 to 1000 Secs

So as you see that there was no significant difference in the Query execution time but there was a 20X increase in the "Fetch Time". Why is this so?

Few other items:
1. The total rows/Columns returned in all cases (pre/post) was identical at ~10 Million Records

2. Inno_db_buffer_pool_size in both case was set to be 2GB

Any guidance is appreciated.

Regards
Deepak

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