What is the change?

At bootstrap, in addition to retrieving table's primary keys and column information, this change adds functionality to retrieve foreign key information

• all the foreign key names in the tables to be exposed
• for each foreign key,
  -- the table it references,
  -- the referenced columns and
  -- the columns from the referencing table.

Why is this change needed?

We need to know the foreign key information since this is useful to write the correct queries to perform joins in GraphQL.

How is this implemented?

• Added a new function SqlMetadataProvider::PopulateForeignKeyDefinitionAsync - takes in all the tables for which foreign key information is to be populated, invokes the methods to retrieve the information and fills in the ForeignKeyDefinition for each table. SqlGraphQLFileMetadataProvider calls this function at bootstrap
• For PostgreSql, MsSql the query to get foreign key information is same and requires a 3-fold join, added that to the BaseSqlQueryBuilder.
  For MySql, the foreign key query only needs the view KEY_COLUMN_USAGE - added it to MySqlQueryBuilder
• To filter for schema, we need to use database name as the value for TABLE_SCHEMA in MySql
• For faster bootstrap, we issue a single query to the database and get information for all tables in one shot.

How is this tested?

• Removed the foreign key information from sql-config.json and made sure existing tests don't regress.
• Added a new assert to validate we retrieve the expected foreign key information in the SqlMetadataProviderTest. To know what we expect, made sure sql-config-test.json is updated.
• Added a case where foreign key contains multiple referenced/referencing columns - see stocks_price_stocks_fk
• Added a case where foreign key references a non-primary key unique column - see stocks_comics_fk

Remarks

• After this change, the only additional information in the sql-config.json as compared to cosmos-config.json is the HttpVerbs section which can also be removed in favor of relying on sql-config-test.json for testing until the developer config change is checked in.
• We can now generate a GQL schema from all the metadata information we have retrieved from database.