CockroachDB session settings for migrations

[tomhoule]

In its current state, Migrate relies on the following session variables for diffing to function properly:

SET enable_experimental_alter_column_type_general = true;
SET default_int_size = 4;
SET serial_normalization = 'sql_sequence';

These are session variables that are set on a per-connection basis and not persisted on the server. The same options can be set to the same or different values with SET CLUSTER SETTING.

Let's review what they do:

• enable_experimental_alter_column_type_general allows migrating the type of a column in an ALTER TABLE statement, with restrictions (depending on column types, and whether the column is indexed). It makes the experience better, and we count on this becoming the default behaviour, so this is fairly unproblematic.
• default_int_size makes INTEGER map to INT4, i.e. a 32-bit integer instead of the default 64 bit. This is enabled to align with postgresql, on which our diffing logic is based. We could change that, it's just more convenient for our tests and our internal diffing logic.
• serial_normalization = 'sql_sequence' ensures we can have SERIAL columns that are not BigInt, since that's implicit in the default value of rowid. Without this, the "traditional" Prisma id Int @default(autoincrement()) is not possible (it will always be a BigInt).

Open questions

• Do we want to handle any combination of server settings? For example in case of default_int_size, we would need to adjust the diffing logic based on default_int_size. More problematic, for serial_normalization, we would either reject valid schemas during prisma schema validation, or allow schemas that will not work on all servers (it depends on the server setting, which we don't know until we have connected).
• Assuming we do not want to support any combination of server settings, we will use session variables like we do now. Is it significantly better to stick to the cockroachdb defaults in the value of these variables?
• Do we want to avoid ambiguous column types like INTEGER and always use the cockroach-specific INT2, INT4 and INT8 types for cockroachdb migrations?

[janpio]

Context:
https://github.com/prisma/prisma-engines/blob/fe68ee16ec7694924b82bf075382fa68e69989da/migration-engine/connectors/sql-migration-connector/src/flavour/postgres.rs#L29-L35

It seems default_int_size has already been removed via prisma/prisma-engines@918d142