Account for derived objects when comparing schemas - #7477
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A DerivedObject describes, without a name, a schema object a platform creates on its own — an index enforcing a constraint, or a constraint a unique index carries. Each platform's provider says which objects it derives from a table's declared ones.
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The comparator asks the platform's provider which objects the database adds to a table, so it can tell them apart from the ones the application declared.
The comparator expects the index the engine creates over a foreign key's referencing columns, so the definition layer no longer mirrors it with an invented one.
Comparing schemas requires telling whether two unique constraints describe the same constraint.
The get*ForAlteredTable() helpers return the objects to create the rebuilt table with.
The method takes how a constraint reports its columns and how it is rebuilt with new ones, so a second kind of constraint can reuse it.
A table diff carries the unique constraints to add and the names of the ones to drop, and each platform renders them.
The method takes how two constraints of a kind compare, so a second kind can reuse it. A constraint that matches no longer goes on being compared with the rest.
The comparator tells the unique constraints of two tables apart, so a table diff carries the ones to add and drop. The method that does it serves the foreign key constraints as well.
Introspection describes what the database has. A table with a primary key or a unique constraint has a unique index over its columns, so the per-platform predicates that withheld it are gone. The comparator expects each such index, so reporting it does not turn a round trip from the database towards the declared schema into a difference.
The comparison from the declared schema to the one the database reports is no longer supported.
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