What is Optimizer Trace? After reading topic about Optimizer Tracer by [Morgan Tocker][1] decided to test it. From [Optimizer Trace and EXPLAIN FORMAT=JSON in 5.7][2]: Optimizer trace is a new diagnostic tool introduced in MySQL 5.6 to show how the optimizer is working internally. It is similar to EXPLAIN, with a few notable differences: It doesn’t just show the intended execution plan, it shows the alternative choices. You enable the optimizer trace, then you run the actual query. It is far more verbose in its output. For understanding goal of article please read previous one about related verified optimizer BUG: [Playing with count() optimizer work][3] ** We have 2 queries: **select count() from sales; select count(*) from sales where sales_id > 0; Firstly let’s get explain plan for query with JSON format and as regular:
-- JSON
mysql> explain format=json select …