The reason behind this change is that MariaDB 10.x does not take into
account that for engines like InnoDB, that scanning a primary key can be up
to 7x faster than scanning a secondary key + read the row data trough the
primary key.

optimizer-adjust_secondary_key_costs is added to provide 2 small
adjustments to the 10.x optimizer cost model. This can be used in the
case where the optimizer wrongly uses a secondary key instead of a
clustered primary key.

The different values for the new variable:

optimizer_adjust_secondary_key_costs=0 (default)

• No changes to current model

optimizer_adjust_secondary_key_costs=1

• Ensure that the cost of of secondary indexes has a cost of at
  least 5x times the cost of a primary key.
  This disables part of the worst_seek optimization described below.

optimizer_adjust_secondary_key_costs=2

• Disable "worst_seek optimization and adjust filter cost slightly
  (add cost of 1 if filter is used).