Mysql 5.7 vs 5.5: simple query takes 50 times longer to run

I have a database "algebra" with a table "questions" with 1,033,990 rows. The records have an attribute 'solved' that is INT. I have a simple query

select count(*) from questions where solved = 0

I have two servers with similar CPUs. On both servers the tables are the same. (database is a replica of production). They are on SSDs. One server has Ubuntu 14.04 with MySQL 5.5.49, and another server has Ubuntu 16.04 with MySQL 5.7.12.

The problem is that this query takes only 0.009s on MySQL 5.5, but takes 0.304s on MySQL 5.7. Which is 34 times SLOWER!!!

The query plans are roughly similar:

Slow server:

id      select_type     table   partitions      type    possible_keys   key     key_len ref     rows    filtered        Extra
1       SIMPLE  questions       NULL    index   NULL    by_topic_solved 97      NULL    1033990 10.00   Using where; Using index

Fast Server:

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   SIMPLE  questions   index   NULL    by_topic_solved 97  NULL    1033989 Using where; Using index

Why this could be the case baffles me. I have seen other, more complicated performance problems with this new 5.7 setup, but this is the simplest problem to be tackling in terms of the underlying query.

I am grasping to find the explanation for this or where to start. my.cnf was roughly similar between the two. Any suggestions would be gratefully accepted.

Solution 1:

I have fully understood this issue. What changed is that in the new mysql, query cache is disabled due to default setting of query_cache_type = OFF. When I re-enabled query cache by setting query_cache_type=1, better performance came right back. Since my site uses 100 times more SELECT queries than INSERT/UPDATE queries, using query cache makes sense for me. Thanks for pointing me to look in this direction!