SQL performance optimization-is the mysql subquery executed repeatedly?

Ask! When mysql returns a fixed result set, whether the result set is repeatedly executed, for example, 1: selectt. *, (selectcount (1) fromtable01) fromtable01t; for example, 2: selectt. *, (selectcount (1) fromtable01t2wheret2. keyt1.key) fromtable01t1; -- Nested subqueries like this will be executed repeatedly. Q: In the subquery traversal process in example 1 above, there are fixed result sets. Will the result be repeated for N times (N records )? How to check the performance of... mysqlsql in the execution plan

Ask! Whether the fixed result set returned by mysql is repeatedly executed
Example 1: select t. *, (select count (1) from table01) from table01 t;

For example, 2: select t. *, (select count (1) from table01 t2 where t2.key = t1.key) from table01 t1; -- Nested subqueries like this will be executed repeatedly.
Q: In the subquery traversal process in example 1 above, there are fixed result sets. Will the result be repeated for N times (N records )?
In the execution plan, how does one check whether the subquery has been executed repeatedly or analyze the execution efficiency of the subquery!