[
https://issues.apache.org/jira/browse/TINKERPOP-2753?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17554861#comment-17554861
]
Boxuan Li commented on TINKERPOP-2753:
--------------------------------------
> Do you find that {{noop()}} is different from {{identity()}} in terms of what
> it does?
Yeah, I think they will basically be the same, except I would assume the
`noop()` step cannot be used as a terminal step. The semantics are the only
notable differences between `noop` and `identity`.
What you offered is indeed a workable solution. In fact, I think it has the
same semantic meaning as our current workaround, `map(x -> x.get())`. I would
still consider it as a workaround, though. But yeah, if the use case I
described is rare, and especially if no other graph provider has the same
problem, then it might not be worth adding a new step.
> Create noop() step to avoid eager optimization
> ----------------------------------------------
>
> Key: TINKERPOP-2753
> URL: https://issues.apache.org/jira/browse/TINKERPOP-2753
> Project: TinkerPop
> Issue Type: Improvement
> Components: process
> Affects Versions: 3.6.0
> Reporter: Boxuan Li
> Priority: Major
>
> 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:
>
> {code:java}
> g.V().has("name", "bob").has("age", 20){code}
>
> 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:
>
> {code:java}
> g.V().has("name", "bob").map(x -> x.get()).has("age", 20){code}
>
> 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:
>
> {code:java}
> g.V(ids).bothE("follows").noop().where(__.otherV().is(v2)).next(){code}
>
> 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) might 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.
--
This message was sent by Atlassian Jira
(v8.20.7#820007)
