Refactor `ProvenanceStoragePostgreSql`
Move PosgreSQL's backend method relation_get
to SQL function
Refactor relation_add
prepearing to move SQL queries into functions
The four flavor's cases are made explicit to simplify the reading and the definition of future SQL functions.
Move PosgreSQL's backend method relation_add
to SQL function
Improve denormalized PostgreSQL schema to properly check for duplicates
Depends on !72 (closed).
Migrated from D6090 (view on Phabricator)