Ignore primary key, foreign key and unique constraints for DDL generation

[willmostly]

Description

Define custom visitors in TrinoDDLCompiler to ignore constraints.

Non-technical explanation

By default, SQLAlchemy adds PRIMARY KEY and other constraint keywords to table creation DDL when these constraints are defined on a Table or class derived from declarative base. Since Trino does not support constraints, this results in invalid SQL. With this change the trino sqlalchemy extension now ignores primary key, foreign key and unique constraints when generating table DDL.

Release notes

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* ignore constraints for table DDL generation

[damian3031]

Rather than silently ignoring constraints, maybe we should raise an explicit error indicating that these constraints are not supported in Trino:

Suggested change

	def visit_foreign_key_constraint(self, constraint, **kw):
	return None
	from sqlalchemy.exc import CompileError
	...
	def visit_foreign_key_constraint(self, constraint, **kw):
	raise CompileError("Trino does not support FOREIGN KEY constraints.")

---

Or at least we could add a warning to make it clearer that the constraint is being ignored:

Suggested change

	def visit_foreign_key_constraint(self, constraint, **kw):
	return None
	import warnings
	from sqlalchemy.exc import SAWarning
	...
	def visit_foreign_key_constraint(self, constraint, **kw):
	warnings.warn("Trino does not support FOREIGN KEY constraints. Constraint will be ignored.", SAWarning)
	return None

@willmostly @hashhar WDYT?