Feature request: schemadiff to be aware and knowledgeable about INSTANT-capable changes

[shlomi-noach]

We would like schemadiff, and in particular the SchemaDiff object (which is the result of a Diff operation between two schemas) to be able to tell whether the set of diffs is applicable in an immediate/instant operation. We consider all but ALTER TABLE to be applicable immediately (e.g. CREATE TABLE, ALTER VIEW take almost no time to run). For an ALTER TABLE, we want schemadiff to judge the change combined by the original schema, and conclude whether it's applicable via ALGORITHM=INSTANT.

Today, there is logic in Online DDL to draw a somewhat similar conclusion. However, the Online DDL analysis takes place only after migrations are submitted. Having this logic in the declarative schemadiff library means we can make decisions before actually submitting the changes.

Eventually, we will want to refactor the existing Online DDL analysis logic to use schemadiff .