mysql query optimization strategy on single table covering index - mysql

Question

currently i am working on mysql query optimization . My mysql table contain 200 million records.
After doing lots of google i finally decide to use covering index.
So, i made a index in this order
alter table table_name add index
index_name(MODEL, COUNTRY, REGION, NETWORK, E_TUAL,
ECHS, DEVID, COUNTRY_CODE, SOURCE);
when i run this query efficiency is good as compare to previous
SELECT E_TUAL,ECHS, DEVID, MODEL, COUNTRY, REGION, COUNTRY_CODE, NETWORK, SOURCE
FROM table_name
WHERE model = 'fox | s453' AND country = 'india' AND
E_TUAL <= '1435755600' AND
E_TUAL >= '1433163600'
ORDER BY E_TUAL DESC
LIMIT 101 OFFSET 0;
+----+-------------+-----------+------+---------------+--------+---------+-------------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+--------+---------+-------------+------+-----------------------------+
| 1 | SIMPLE | mytable | ref | genrl | genrl | 131 | const | 239 | Using where; Using filesort |
+----+-------------+-----------+------+---------------+--------+---------+-------------+------+-----------------------------+
But This query is worst compare to older one (before index)
SELECT E_TUAL,ECHS, DEVID, MODEL,COUNTRY, REGION, COUNTRY_CODE, NETWORK, SOURCE
FROM table_name
WHERE model = 'fox | s453' AND
country is not null AND
E_TUAL <= '1435755600' AND
E_TUAL >= '1433163600'
ORDER BY E_TUAL DESC
LIMIT 101 OFFSET 0;
+----+-------------+-----------+-------+---------------+--------+---------+------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+-------+---------------+--------+---------+------+------+-----------------------------+
| 1 | SIMPLE | mytable | range | genrl | genrl | 131 | NULL | 1105 | Using where; Using filesort |
+----+-------------+-----------+-------+---------------+--------+---------+------+------+-----------------------------+
i write country is not null to maintain order so mysql optimizer use index
plz help how to improve efficiency when some of column field is absent and
to make use of index what can i do ?
I am not good in english. so for any mistake i am sorry

Answer 1

Dispense with the covering index and use two separate indexes:
alter table table_name add index index_name(MODEL, COUNTRY, E_TUAL);
for the first query. And:
alter table table_name add index index_name(MODEL, E_TUAL);
A covering index might provide marginal improvement, but it will use a lot of space. Instead, focus on the WHERE clauses and then the ORDER BY if you can use those for the index.
As a note: your ordering of the columns was not optimal for either query.