Re: Explain plans for InnoDB and Memory Engines

[Rick James]

Did both EXPLAINs say "Using index", thereby indicating a "covering" index. That is, the query was performed in the index, without looking at the data.

INDEX(col15,col1) is not the best index for
SELECT DISTINCT col15 from table_1 WHERE col1=2
Instead, have
INDEX(col1,col15)

Your query had to scan the entire index.

Please provide SHOW CREATE TABLE

Note that InnoDB really likes to have a PRIMARY KEY.

Your choice of transactional BEGIN...COMMIT can make a big difference in InnoDB performance. Did you run without such? With what setting of autocommit?

And what about innodb_flush_log_at_trx_commit?