custom_sql fails for Redshift databases

[elsander]

Redshift unfortunately does not have functionality for IF NOT EXISTS, so the temporary table creation fails when passing custom sql to a Redshift SQLAlchemy engine. It seems like the options are here are to introspect the engine and special-case Redshift (seems harder to maintain), to remove the IF NOT EXISTS for everyone and let the custom sql fail if the table exists (I think I'd expect this behavior as a user anyway), or to not support custom_sql for Redshift.

[elsander]

If the maintainers have thoughts on which approach makes the most sense for the library, I'm happy to take a stab at this next week as well. I'm hoping to make some contributions to this library for Hacktoberfest :)

[ccnobbli]

Cool! I haven't looked into it but the second option makes sense to me. @jpcampb2 is the author and resident expert on the SQLAlchemy implementations.