MySQL modifies SQL statements to optimize performance

The phenomenon and background of the problem

MySQL select fields a, b from (left join multi-table connection) m, (left join multi-table connection) n where m. field = n. field, because m table has no where condition query is very slow, I want to optimize SQL.
How to write SQL statements, select fields a,b from (left join multi-table connection where m.field=n.field from (left join multi-table connection) n)m, can you find a simple example
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