Query: shortcircuit clause resolution when result is guaranteed to be…

… empty.

The `-collect` fn, run at the end of a query, correctly shortcircuits with the
following comment: "one empty rel means final set has to be empty"

However, while processing the query clauses, there was no shortcircuit.
Also, since the implementation of `lookup-pattern` is agnostic to
existing rels, this means that attempts to write short-circuiting
clauses at the top of a complex query have very little effect: the
query engine will continue querying the indexes for every tuple that
could possibly match every clause.

This adds a shortcircuit in `resolve-clause` if any existing relation
is empty. The check is cheap, and it should provide a substantial
performance boost in many common cases.

bench/datascript/bench/datascript.cljc

@@ -110,6 +110,19 @@
                   [?e :sex :male]]
       @*db100k)))
 
+(defn bench-q5-shortcircuit []
+  (bench/bench
+    (d/q '[:find ?e ?n ?l ?a ?s ?al
+           :in $ ?n ?a
+           :where [?e :name ?n]
+                  [?e :age ?a]
+                  [?e :last-name ?l]
+                  [?e :sex ?s]
+                  [?e :alias ?al]]
+      @*db100k
+      "Anastasia"
+      35)))
+
 (defn bench-qpred1 []
   (bench/bench
     (d/q '[:find ?e ?s
@@ -226,6 +239,7 @@
    "q2"                 bench-q2
    "q3"                 bench-q3
    "q4"                 bench-q4
+   "q5-shortcircuit"    bench-q5-shortcircuit
    "qpred1"             bench-qpred1
    "qpred2"             bench-qpred2
    "pull-one-entities"  bench-pull-one-entities
@@ -277,6 +291,7 @@
   (bench-q2)
   (bench-q3)
   (bench-q4)
+  (bench-q5-shortcircuit)
   (bench-qpred1)
   (bench-qpred2)
   (bench-pull-one-entities)
@@ -298,4 +313,4 @@
   (bench-rules-long-30x3)
   (bench-rules-long-30x5)
   (bench-freeze)
-  (bench-thaw))
+  (bench-thaw))

src/datascript/query.cljc

@@ -791,12 +791,14 @@
          (update context :rels collapse-rels relation))))))
 
 (defn resolve-clause [context clause]
-  (if (rule? context clause)
-    (if (source? (first clause))
-      (binding [*implicit-source* (get (:sources context) (first clause))]
-        (resolve-clause context (next clause)))
-      (update context :rels collapse-rels (solve-rule context clause)))
-    (-resolve-clause context clause)))
+  (if (->> (:rels context) (some (comp empty? :tuples)))
+    context ; The result is empty; short-circuit processing
+    (if (rule? context clause)
+      (if (source? (first clause))
+        (binding [*implicit-source* (get (:sources context) (first clause))]
+          (resolve-clause context (next clause)))
+        (update context :rels collapse-rels (solve-rule context clause)))
+      (-resolve-clause context clause))))
 
 (defn -q [context clauses]
   (binding [*implicit-source* (get (:sources context) '$)]