The script will generate the missing opencart indexes. Copy the result and execute it on the target schema.

opencart_fix_indexes.sql

use information_schema;

select
  concat('alter table `', c.table_name, '` add index `ix_', c.column_name, '` (`',c.column_name,'`);')
from columns c
where c.table_schema = 'SCHEMA_NAME'
and lower(c.column_name) like '%_id%'
and c.column_key = '';