Add t value to pattern resolution

[quoll]

Datomic expects to bind a 4th element to a transaction "t" value.
e.g.
[:entity :attr ?value ?t]

To do this, the basic graph patterns will need to result in a t-value lookup in the indexes. These are currently stored in the durable system in another index (and will require a join), and not stored at all in the in-memory index (ticket #198).

The current graph API has resolve-triple with 4 arguments (graph, subject, predicate, object).

We should consider new arities here, though a simple extension to a 5th argument will cause problems with considering bindings of statement IDs. The new arity may need to accept 6 arguments.

When considering the new arities with respect to #200 we are looking for the following mappings of BGPs:

User Syntax	Internal Representation
[?e ?a ?v]	[?e ?a ?v _ _]
[?e ?a ?v ?t]	[?e ?a ?v _ ?t]
[?e ?a ?v :as ?stmt]	[?e ?a ?v ?stmt _]
[?e ?a ?v ?t :as ?stmt]	[?e ?a ?v ?stmt ?t]

There are 3 cases to consider:

• The t value is variable: Lookup the value and return in bindings.
• The t is set, along with other values: The t value is then used as a filter (directly on statements for in-memory, and via the tSPO index for durable).
• The t is set, and everything else is variable: Filter for in-memory, but use the tSPO index for durable storage.

Depends on t values from #198

[quoll]

Initial data

(def d1 [[:a :p 1] [:b :p 2]])  ;; transaction 1
(def d2 [[:c :p 3] [:d :p 4]])  ;; transaction 2
(transact c {:tx-triples d1})
(transact c {:tx-triples d2})

T value is variable

Querying for transaction IDs:

=> (q '[:find ?e ?v ?t :where [?e :p ?v ?t]] (as-of (db c) 1))
([:a 1 1] [:b 2 1])
=> (q '[:find ?e ?v ?t :where [?e :p ?v ?t]] (as-of (db c) 2))
([:a 1 1] [:b 2 1] [:c 3 2] [:d 4 2])

Performed by looking up the queries as [?e :p ?v], then:

• in memory: emit the :t value
• durable: lookup the tspo index, and emit column 0

T value is set. Other values set

Querying using transaction IDs. Predicate is set to :p:

=> (q '[:find ?e ?v :where [?e :p ?v 1]] (db c))
([:a 1] [:b 2])
=> (q '[:find ?e ?v :where [?e :p ?v 2]] (db c))
([:c 3] [:d 4])
=> (q '[:find ?e ?v :where [?e :p ?v 2]] (as-of (db c) 1))
()

Performed by looking up the queries as [?e :p ?v], then:

• in memory: filter by :t value
• durable: lookup the tspo index, filter by column 0

T value is set. All other values variable

Querying using transaction IDs.

=> (q '[:find ?e ?p ?v :where [?e ?p ?v 1]] (db c))
([:a :p 1] [:b :p 2])
=> (q '[:find ?e ?p ?v :where [?e ?p ?v 2]] (db c))
([:c :p 3] [:d :p 4])
=> (q '[:find ?e ?v :where [?e :p ?v 2]] (as-of (db c) 1))
()

Performed by:

• in memory: Iterate over every triple in SPO. Filter by transaction value
• durable: Binary search the tspo index on column 0 for the t value, and t+1.
  This gives 2 longs: lower-bound, upper-bound.
  Result is: (map (fn [id] (subvec (get-record tspo id) 1)) (range lower-bound upper-bound))