Graql Delete queries should accept statements

[haikalpribadi]

Problem to Solve

This issue actually addresses several issues.

1. The ability to remove attributes from its owners without deleting the attribute itself
2. The ability to remove roleplayers from a relation

Current Workaround

1. In order to delete an attribute from its owners right now uses the via keyword.

   match $x isa person, has name $n via $h; delete $h;
   

   However, we will be removing the via keyword, as we will be deleting the implicit relation that exists to implement the has keyword. Instead, we will have native has data structure.

2. Right now, there is no workaround to remove a roleplayer from a relation and this is actually a long-standing issue: Graql: Enable removing roleplayer from relationship in Graql #39.

Proposed Solution

Allow the delete query to operate on full statements, just like insert statements. For example, here's an insert query operating on a statement:

match $x isa person; insert $x has name "alice";

A delete query operating on a statement can therefore do:

match $x isa person, has name $n; delete $x has name $n;

This will result in symmetry of insert and delete queries.

The above example shows how we solve the first problem - removing attributes from their owners. In order to delete roleplayers` from their relation, you can write

match $r (role-a: $x, role-b: $x) isa relation;

delete $r (role-a: $x);

Note that in the examples above, we are not deleting the concept instances represented by the variables themselves. In order to delete the concept instances, you must write:

match $x isa person, has name $n;
delete $x isa person; $n isa name;

This will delete all instances of person and name found in the matched pattern.

Concretely, the following will be implemented:

• we only allow match...delete queries (delete... only clauses are disallowed in the grammar)
• variables in the delete clause must be a subset of the match clause
• allowed deletion statements will by the same set allowed by insert
• as with inserts, using NEQ, Predicates (ie. operations) and ID properties in the delete clause should throw
• To verify that the concepts found in the match clause satisfy the delete clause as well: this means we can satisfy inheritance contraints naturally and throw otherwise, for example in :

match $x isa thing;
delete $x isa person;

however, the following will never throw:

match $x isa person;
delete $x isa thing;

• Deletion with ID edge case:

match $x id V123;
delete $x isa thing;

should work, however:
* willl throw if V123 happens to be a type
* requires the isa thing clause to delete the instance
* will if trying to write match $x id V123; delete $x id V123;

• Note that as with insert statements, users can easily end up with quadratic complexities, such as:

match $x isa person; $n isa name;
delete $x has name $n;

• Deleting duplicate role players:
  we will define that in delete queries:

match $r (arole: $x) isa reflexive;
delete $r (arole: $x); $r (arole: $x);

should be equivalent to :

match $r (arole: $x) isa reflexive;
delete $r (arole: $x, arole: $x);

this should lend itself to a straightforward implementation of executing one delete operation per property

Additional Information

Implementing the proposed solution above would allow us to introduce an "update query", by doing:

match $x isa person, has first-name "alice", has last-name $n; 
delete $x has last-name $n;
insert $x has last-name "Davidson";

Pieces of Work

• update grammar (Deletes accept statement_instance instead of variables #118)
• modify grakn executors to handle statements (Deletes accept statements instead of variables typedb#5690) (Collect match clause before execute delete typedb#5712)
• prepare documentation (Update for 1.8: delete syntax, remove via, query options, syntax renaming typedb-docs#296)
• remove via keyword, and propagage (Remove VIA #124) (Remove implicit variable statement #125)
• update internal clients and users of delete as required
• delete BDD (Support for Graql Delete and validate keys on deletion of ownership typedb#5711) (Graql Delete Scenarios typedb-behaviour#38)

[flyingsilverfin]

Completed!