如何检查ArangoDB查询是否为空？

Question

I would like to make an exists PostgreSQL query.

Let's say I have a Q ArangoDB query (AQL). How can I check if Q returns any result?

Example:

Q = "For u in users FILTER 'x@example.com' = u.email"

What is the best way to do it (most performant)?

I have ideas, but couldn't find an easy way to measure the performance:

Idea 1: using Length :

RETURN LENGTH(%Q RETURN 1) > 0

Idea 2: using Frist :

RETURN First(%Q RETURN 1) != null

Above, %Q is a substitution for the query defined at the beginning.

Answer 1

Do you need and AQL solution?

Only the count:

var q  = "For u in users FILTER 'x@example.com' = u.email";
var res = db._createStatement({query: q, count: true}).execute();
var ct = res.count();

Is the fastest I can think of.

Answer 2

I think the best way to achieve this for a generic selection query with a structure like

Q = "For u in users FILTER 'x@example.com' = u.email"

is to first add a LIMIT clause to the query, and only make it return a constant value (in contrast to the full document).

For example, the following query returns a single match if there is such document or an empty array if there is no match:

FOR u IN users FILTER 'x@example.com' == u.email LIMIT 1 RETURN 1

(please note that I also changed the operator from = to == because otherwise the query won't parse).

Please note that this query may benefit a lot from creating an index on the search attribute, ie email . Without the index the query will do a full collection scan and stop at the first match, whereas with the index it will just read at most a single index entry.

Finally, to answer your question, the template for the EXISTS -like query will then become

LENGTH(%Q LIMIT 1 RETURN 1)

or fleshed out via the example query:

LENGTH(FOR u IN users FILTER 'x@example.com' == u.email LIMIT 1 RETURN 1)

LENGTH(...) will return the number of matches, which in this case will either be 0 or 1. And it can also be used in filter conditions like as follows

FOR ....
  FILTER LENGTH(...)
  RETURN ...

because LENGTH(...) will be either 0 or 1, which in context of a FILTER condition will evaluate to either false or true.