Remove Preql scripts; rely on database_types.py instead

[sirupsen]

@erezsh wdyt about removing all the Preql scripts and just rely on database_types.py instead? We can add instructions on running a single test to seed your local database if you want to just run it locally.

I think this would cut down on the amount of testing code we have to maintain

[erezsh]

If someone wants to send a PR for it, they're welcome to.

[erezsh]

Things that it would have to solve:

• DDL operations (mostly create & drop, but requires translating column types)
• Resolving table names, so it uses the right schema, handling of lower/upper-case mangling, and avoiding reserved keywords (like comment in oracle)
• Syntax variations (there are many). examples:
  • In MySQL || is "or", not "concat". In Oracle, "concat" can only accept two arguments, but || is concat.
• (to be updated)

So far, trying to get our tests to run on all databases, only makes me think that more of them should use Preql, not less.

[nklsw]

I just stumbled across this while building a driver for Exasol. Is there anything new on this topic? How should one currently deal with testing new drivers for databases that are not supported by Preql?

[erezsh]

@nklsw Preql is not necessary to test new drivers. Read this guide: https://data-diff.readthedocs.io/en/latest/new-database-driver-guide.html

[nklsw]

@erezsh Do I understand you correctly that I should not include a new database in TEST_DATABASES? Just out of interest: What are these tests for and why should not all supported databases be tested there?

[erezsh]

Task completed.