Remember SQL optimization--left join does not go index problem

SQL is stuck on execution, then ... Kill the process.

Look at the size of the table.


The first reaction is to add the index, and then explain looked at what index, the result is very awkward, three tables, only walked an index ... A group of people in that tangle for the hair can not go index.
Inadvertently found that the character encoding of a table is GBK. The other two are UTF8. Online search, only to find that when the table encoding inconsistent when the left join cannot go index ...

Remember SQL optimization--left join does not go index problem