add @unique constraint support in schema for new predicates

[shivaji-kharse]

Partially Fixes #8827
Closes: DGRAPHCORE-206
Docs PR: dgraph-io/dgraph-docs#638

This PR adds support for uniqueness constraint using @unique directive in DQL schema. This unique directive ensures that all values of the predicate are different in a Dgraph Cluster. This completes phase 1, and enables adding a new predicate with unique directive. As part of the phase 2, we will work on adding support for unique directive for existing predicates.

Performance

Live Loader before this change on 21 million dataset took 10m54s whereas after this change took 11m02s. It did not make any significant different to non-unique predicates.

How to Use

You can now specify unique in schema as follows: email: string @unique @index(hash) @upsert .. Now, Dgraph will ensure that no mutation adds a duplicate for the predicate email.

Phase 2 [TODO]

• check if @unique can be added to schema depending upon whether existing data has any duplicates. If the existing data has any duplicates, we do not allow adding the @unique directive and return a query that allows user to identify these UIDs.
• If index computation is in progress, we should not allow mutations with predicates for which @unique is set
• Fix ACL to ensure that we do not end up adding duplicate users
• Ensure that unique constraint is not violated during Bulk loader

[mangalaman93]

A few comments, this looks pretty good to start with. We need to add test for parsing the schema.

[mangalaman93]

We should for now, do not allow adding unique when the predicate already exists. Later on, we can support unique for existing predicate as well.

[mangalaman93]

Looks pretty good to me now. 3 very minor comments. Please ask other people to review.

[harshil-goel]

I think I will need a meeting to go through the various design decisions.

[rderbier]

What is the behavior if you add @unique on an existing predicate and deploy the schema?
Do we reject the schema or just ignore the unique directive?

[mangalaman93]

we reject the schema and the Alter request should return an error.