Why this simple ArangoDB query sometimes takes very long time

Question

I am querying ArangoDb of about 500k document via arangojs.query() with this very simple query

"FOR c IN Entity FILTER c.id == 261764 RETURN c"

It is a node in node-link graph.

But sometimes, it took more than 10 seconds and in the log of arangodb also has warning about query taking too long.Lots of time it happens if new session is used on browser. Is it problem of arangodb or arangojs or my query itself is not optimized?

-------------------Edit----------------------

Added db.explain

Query string:
 FOR c IN Entity FILTER c.id == 211764 RETURN c

Execution plan:
 Id   NodeType                    Est.   Comment
  1   SingletonNode                  1   * ROOT
  2   EnumerateCollectionNode   140270     - FOR c IN Entity   /* full collection scan */
  3   CalculationNode           140270       - LET #1 = (c.`id` == 211764)   /* simple expression */   /* collections used: c : Entity */
  4   FilterNode                140270       - FILTER #1
  5   ReturnNode                140270       - RETURN c

Indexes used:

 none

Optimization rules applied:

 none

Answer 1

As your Explain shows, your query doesn't utilize indices, but does a full collection scan.

Depending on when it finds the match (at the start or the end of the collection) execution times may vary.

See the Indexing chapter for creating indices, and the AQL Execution and Performance chapter howto analyse the output of db._explain()