Array index on ArangoDb

Question

My collection in ArangoDb have objects with this schema (simplified):

{
    "userName": "Billy",
    "email": "[email protected]",
    "logins": [
      {
        "authenticationTokens": [],
        "loginProvider": "Facebook",
        "providerKey": "123",
        "providerDisplayName": null
      }
    ],
    "roles": [],
    "claims": []
}

That's ASPNetCore.Identity implementation on ArangoDb by BorderEast aspnetcore-identity-arangodb

PROBLEM:

To auth using Facebook it uses AQL query

for u in IdentityUser for l in u.logins filter l.loginProvider == "Facebook" && l.providerKey == "123" return u

which works well but does not use any index

Indexes used:
    none

I have tried indices:

db.IdentityUser.ensureIndex({ type: "hash", fields: [ "logins[*].loginProvider", "logins[*].providerKey" ] });
db.IdentityUser.ensureIndex({ type: "hash", fields: [ "logins[*].loginProvider" ] });
db.IdentityUser.ensureIndex({ type: "hash", fields: [ "logins[*].providerKey" ], unique: true });

None of them is used.

Can someone advise how the index should look like for that query?

Answer 1

The issue lies with they query. It needs to be re-written and then it will work. It has to do with using the in comparison operator as discussed in this answer:

"Still the query in 2.8 won't use that index because the array indexes are only used for the IN comparison operator."

So if we change the query we get this:

Query string:
 for u in IdentityUser 
  let l = u.logins[*].loginProvider
  let p = u.logins[*].providerKey
  filter  "google"  in l and "googlekey" in p
  return u

Execution plan:
 Id   NodeType          Est.   Comment
  1   SingletonNode        1   * ROOT
  8   IndexNode            1     - FOR u IN IdentityUser   /* persistent index scan */
  9   CalculationNode      1       - LET #7 = ("googlekey" in u.`logins`[*].`providerKey`)   /* simple expression */   /* collections used: u : IdentityUser */
  6   FilterNode           1       - FILTER #7
  7   ReturnNode           1       - RETURN u

Indexes used:
 By   Type         Collection     Unique   Sparse   Selectivity   Fields                          Ranges
  8   persistent   IdentityUser   false    false            n/a   [ `logins[*].loginProvider` ]   ("google" in u.`logins`[*].`loginProvider`)

Optimization rules applied:
 Id   RuleName
  1   move-calculations-up
  2   remove-unnecessary-calculations
  3   use-indexes
  4   remove-filter-covered-by-index