[jira] [Created] (TINKERPOP-2753) Create noop() step to avoid eager optimization

["Boxuan Li (Jira)" <ji...@apache.org>]

Boxuan Li created TINKERPOP-2753:
------------------------------------

             Summary: Create noop() step to avoid eager optimization
                 Key: TINKERPOP-2753
                 URL: https://issues.apache.org/jira/browse/TINKERPOP-2753
             Project: TinkerPop
          Issue Type: New Feature
            Reporter: Boxuan Li


I only have experience in JanusGraph, so my opinion might be biased and this proposal might not be generalizable to other graph providers:

I propose we create a `noop()` step that does nothing. It is a special step that simply provides a hint for the graph provider. How to interpret it depends on the graph provider, but the usage in my mind is to avoid eager optimization. Sometimes a graph provider can combine different filter steps into a joint condition for better index selection or predicate pushdown. For example, in the query below:

```java

g.V().has("name", "bob").has("age", 20)

```

JanusGraph will fold the two `has` conditions into a joint condition for better index selection. Sometimes, however, users don't want this "eager optimization", likely because they know the distribution of data and prefer doing in-memory filtering for the second `has` condition. They could do this:

```java

g.V().has("name", "bob").map(x -> x.get()).has("age", 20)

```

So that JanusGraph will defer the evaluation of the second condition until the first `has` condition is evaluated. Here, the `map(x -> x.get())` is essentially a noop step. What I am proposing is to use an official `noop()` step to replace this workaround. This `noop` step sounds like a `barrier` step but they do not have the same semantics. The `noop` step is a barrier against constraint look-ahead optimization.

 

Another example usage of `noop` is as follows:

```java

g.V(ids).bothE("follows").noop().where(__.otherV().is(v2)).next()

```

In the above case, we can use `noop` to force the graph provider to compute `bothE` first and then evaluate `where` statement. Otherwise, the graph provider (for example, JanusGraph) will try folding the `where` condition into the `bothE` step for predicate pushdown. Predicate pushdown usually works, but in some scenarios, it is less preferred.

 

I am happy to provide a patch if the community likes this idea.



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