keep and remove PK indexes to improve query performance, and why? - mysql

Question

I am trying to optimize a few stored procedures in a MySQL database. A few columns have more than one index. These columns are used in joins and WHERE clauses.
Which do I keep and which do I remove to improve query performance, and why?
select table_name,index_name,seq_in_index,column_name,non_unique,index_type,comment from information_schema.statistics
where table_name in('suspicious_activity_report','customer_comment_table','employement_status','Country_Master','Country_Master')
order by table_name,column_name

Answer 1

Your query's output shows tables from all the databases on your system. You need to show which database each table is in, like this:
select table_schema, table_name, index_name, seq_in_index,
column_name, non_unique, index_type, comment
from information_schema.statistics
where table_name in( 'suspicious_activity_report',
'customer_comment_table',
'employement_status',
'Country_Master')
order by table_schema, table_name, seq_in_index;
You can also say WHERE ... table_schema = DATABASE() if you want to show information from only the current database.
This will remove the apparently duplicated indexes from your output.
Multi-column indexes are often added to tables to handle specific query patterns. Your output shows only one multicolumn index: country_master(Country_ID, Client_ID). This index is suitable for these patterns, among a few others.
WHERE Country_ID = <constant> AND Client_ID = <constant>
WHERE Country_ID = <constant> AND Client_ID BETWEEN <constant> AND <constant>.
Like #P.Salmon said, before you remove indexes, you need to figure out which query patterns use them. You could do this by removing them in production and observing where performance collapses. But that's unwise and will anger your users.