doc: comments for child query optimization

src/main/frontend/db/rules.cljs

@@ -1,24 +1,45 @@
 (ns frontend.db.rules)
 
 (def rules
+  ;; rule "parent" is optimized for child node -> parent node nesting queries
   '[[(parent ?p ?c)
      [?c :block/parent ?p]]
     [(parent ?p ?c)
      [?c :block/parent ?t]
      (parent ?p ?t)]
 
+  ;; rule "child" is optimized for child node -> parent node nesting queries
     [(child ?p ?c)
      [?c :block/parent ?p]]
     [(child ?p ?c)
      [?t :block/parent ?p]
      (child ?t ?c)]
 
+  ;; rule "namespace" is optimized for child node -> node of upper namespace level nesting queries
     [(namespace ?p ?c)
      [?c :block/namespace ?p]]
     [(namespace ?p ?c)
      [?t :block/namespace ?p]
      (namespace ?t ?c)]
 
+    ;; Select rules carefully, as it is critical for performance.
+    ;; The rules have different clause order and resolving directions.
+    ;; Clause order Reference:
+    ;; https://docs.datomic.com/on-prem/query/query-executing.html#clause-order
+    ;; Recursive optimization Reference:
+    ;; https://stackoverflow.com/questions/42457136/recursive-datalog-queries-for-datomic-really-slow
+    ;; Should optimize for query the decendents of a block
+    ;; Quote:
+    ;; My theory is that your rules are not written in a way that Datalog can optimize for this read pattern - probably resulting in a traversal of all the entities. I suggest to rewrite them as follows:
+    ;; [[(ubersymbol ?c ?p) 
+    ;;   (?c :ml/parent ?p)]
+    ;;  [(ubersymbol ?c ?p) 
+    ;;   ;; we bind a child of the ancestor, instead of a parent of the descendant
+    ;;   (?c1 :ml/parent ?p)
+    ;;   (ubersymbol ?c ?c1)]]
+
+    ;; This way of writing the ruleset is optimized to find the descendants of some node. The way you originally wrote it is optimized to find the anscestors of some node.
+
     ;; from https://stackoverflow.com/questions/43784258/find-entities-whose-ref-to-many-attribute-contains-all-elements-of-input
     ;; Quote:
     ;; You're tackling the general problem of 'dynamic conjunction' in Datomic's Datalog.
@@ -35,18 +56,4 @@
     ;;            [(identity ?vs) [?v ...]]
     ;;            (not-join [?e ?v]
     ;;                      [?e ?a ?v]))]
-
-    ;; Recursive optimization
-    ;; https://stackoverflow.com/questions/42457136/recursive-datalog-queries-for-datomic-really-slow
-    ;; Should optimize for query the decendents of a block
-    ;; Quote:
-    ;; My theory is that your rules are not written in a way that Datalog can optimize for this read pattern - probably resulting in a traversal of all the entities. I suggest to rewrite them as follows:
-    ;; [[(ubersymbol ?c ?p) 
-    ;;   (?c :ml/parent ?p)]
-    ;;  [(ubersymbol ?c ?p) 
-    ;;   ;; we bind a child of the ancestor, instead of a parent of the descendant
-    ;;   (?c1 :ml/parent ?p)
-    ;;   (ubersymbol ?c ?c1)]]
-    
-    ;; This way of writing the ruleset is optimized to find the descendants of some node. The way you originally wrote it is optimized to find the anscestors of some node.
     ])