View the original community article here
Last tested: Nov 5, 2019
For most dialects a scratch schema is not needed in order to use non-persistent derived tables.
For the MySQL family (Clustrix, MemSQL, MariaDB) you do need to do some additional setup to allow ephemeral derived tables. According to docs, you can just grant the proper permissions to a schema that doesn't necessarily exist.
CREATE TEMPORARY TABLES
-- Note that the looker_tmp schema does not need to actually exist,
-- even though these permission grants are still needed