Peformance behaviour MySQL 5.7 vs. MySQL 8

Hi,

I recently upgraded from MySQL 5.7 to MySQL 8, and observed that the same query is performing much slower after the upgrade.

Please see example below..

I have table psa and table rmv. The query is like,

SELECT * FROM psa left join rmv on psa.pein = rmv.pein

Both psa and rmv tables have index KEY `pein` USING BTREE (`pein`).

The same query, it only takes less than 1 second in my old 5.7 database, while it takes around 15 minutes in new database.

And when I tried to do EXPLAIN on the query, it is telling different JOIN type..

-- EXPLAIN query in 5.7