MySQL- Improvement on count(*) aggregation with composite index keys - mysql

Question

I have a table with the following structure with almost 120000 rows,
desc user_group_report
+------------------+----------+------+-----+-------------------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------+----------+------+-----+-------------------+-------+
| user_id | int | YES | MUL | NULL | |
| group_id | int(11) | YES | MUL | NULL | |
| type_id | int(11) | YES | | NULL | |
| group_desc | varchar(128)| NO| | NULL |
| status | enum('open','close')|NO| | NULL | |
| last_updated | datetime | NO | | CURRENT_TIMESTAMP | |
+------------------+----------+------+-----+-------------------+-------+
I have indexes on the following keys :
user_group_type(user_id,group_id,group_type)
group_type(group_id,type_id)
user_type(user_id,type_id)
user_group(user_id,group_id)
My issue is I am running a count(*) aggregation on above table group by group_id and with a clause on type_id
Here is the query :
select count(*) user_count, group_id
from user_group_report
where type_id = 1
group by group_id;
and here is the explain plan (query taking 0.3 secs on average):
+----+-------------+------------------+-------+---------------------------------+---------+---------+------+--------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------------+-------+---------------------------------+---------+---------+------+--------+--------------------------+
| 1 | SIMPLE | user_group_report | index | user_group_type,group_type,user_group | group_type | 10 | NULL | 119811 | Using where; Using index |
+----+-------------+------------------+-------+---------------------------------+---------+---------+------+--------+--------------------------+
Here as I understand the query almost does a full table scan because of complex indices and When I am trying to add an index on group_id, the rows in explain plan shows a less number (almost half the rows) but the time taking for query execution is increased to 0.4-0.5 secs.
I have tried different ways to add/remove indices but none of them is reducing the time taken.
Assuming the table structure cannot be changed and querying is independent of other tables, Can someone suggest me a better way to optimize the above query or If i am missing anything here.
PS:
I have already tried to modify the query to the following but couldn't find any improvement.
select count(user_id) user_count, group_id
from user_group_report
where type_id = 1
group by group_id;
Any little help is appreciated.
Edit:
As per the suggestions, I added a new index
type_group on (type_id,group_id)
This is the new explain plan. The number of rows in explain,reduced but the query execution time is still the same
+----+-------------+------------------+------+---------------------------------+---------+---------+-------+-------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------------+------+---------------------------------+---------+---------+-------+-------+--------------------------+
| 1 | SIMPLE | user_group_report | ref | user_group_type,type_group,user_group | type_group | 5 | const | 59846 | Using where; Using index |
+----+-------------+------------------+------+---------------------------------+---------+---------+-------+-------+--------------------------+
EDIT 2:
Adding details as suggested in answers/comments
select count(*)
from user_group_report
where type_id = 1
This query itself is taking 0.25 secs to execute.
and here is the explain plan:
+----+-------------+------------------+------+---------------+---------+---------+-------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------------+------+---------------+---------+---------+-------+-------+-------------+
| 1 | SIMPLE | user_group_report | ref | type_group | type_group | 5 | const | 59866 | Using index |
+----+-------------+------------------+------+---------------+---------+---------+-------+-------+-------------+

Answer 1

I believe that your group_type is wrong. Try to switch the attributes.
create index ix_type_group on user_group_report(type_id,group_id)
This index is better for your query because you specify the type_id = 1 in the where clause. Therefore, the query processor finds the first record with type_id = 1 in your index and then it scans the records in the index with this type_id and performs the aggregation. With such index, only relevant records in the index are accessed which is not possible with the group_type index.

Answer 2

If type_id is selective (i.e. it reduces the search space significantly), creating an index on type_id, group_id should help significantly.
This is because it reduces the number of records that need to be grouped first (remove everything where type_id != 1), and only then does the grouping/summing.
EDIT:
Following on from the comments, it seems we need to figure out more about where the bottleneck is - finding the records, or grouping/summing.
The first step would be to measure the performance of:
select count(*)
from user_group_report
where type_id = 1
If that is significantly faster, the challenge is likely in the grouping than in finding the records. If that's just as slow, it's in finding the records in the first place.

Answer 3

Do most of the columns really need to be NULLable? Change to NOT NULL where applicable.
What percentage of the table has type_id = 1? If it is most of the table, then that would explain why you don't see much improvement. Meanwhile, the EXPLAIN seems to be thinking there are only two distinct values for type_id, hence it says only half the table will be scanned -- this number cannot be trusted.
To get more insight into what is going on, please do these:
EXPLAIN FORMAT=JSON SELECT...;
And
FLUSH STATUS;
SELECT ...
SHOW SESSION STATUS LIKE 'Handler%';
We can help interpret the data you get there. (Here is a brief discussion of such.)