Hi Andrii,
Thanks for starting this discussion and putting together this proposal. I want
to start by saying that overall, I'm massively in favour of the proposed
overhaul of match(). This is a topic that has come up many times in the past,
and taking advantage of an established declarative language like GQL always
seems to be the preferred solution.
The idea of having the language configurable via something like
`.with(“language”,
“GQL”)` is quite interesting, and something I haven't seen in previous
discussions. There is clear value in allowing providers to support their own
preferred declarative languages here, but I also worry about the loss of query
portability if TinkerPop is too hands off on the choice of declarative
language. I believe the vast majority of usages here will be seeing a traversal
with a simple GQL-like match pattern. I think it would make sense for TinkerPop
to adopt a default language for the new match step, which is some heavily
restricted form of GQL (read-only, limited to basic MATCH, WHERE, and RETURN
statements). This "standard" language could then be used in the new match step
without a language with-modulator. Providers would still be free to support
their own languages via that modulator if they choose.
I will take a bit more time to consider the withParameter() proposal. My
initial reaction is that I prefer to tie it into the existing parameter
bindings included in remote requests to gremlin-server. I would like query
parameters to function in a unified manner across the entire traversal if
possible, instead of a separate detached system isolated to the new match step.
I understand the current limitation of only supporting parameters in remote
traversals. I'm not immediately seeing the need to support parameters for
embedded traversals here, I'd be interested if you have any examples where
embedded parameters present a clear advantage. If we do decide there is a need
for embedded parameters, I would prefer to solve that problem at the broader
gremlin level, instead of isolating it to the match step.
I totally agree that the start and mid-step behaviour of the new match step
should be modeled after V() and E().
I think the trickiest part of getting this right is the return types. The most
common use cases I expect is where the RETURN clause only includes a single
node or edge. In this case I completely agree with returning the element
itself. I definitely want to support usages such as g.match("MATCH
(n{name:'Cole'}) RETURN n").out()... My main tenet here is that results should
naturally flow from the declarative match into the subsequent gremlin and be
easy to consume. If multiple objects are returned, I would agree that it is
necessary to return a Map<String, ?> as in g.match("MATCH
(p:person)-[e:created]->(s:software) RETURN *") -> {"p": V[1], "e": E[9], "s":
V[3]} ...
I'm still on the fence for how to handle single returns of non-elements. I see
the value in your recommendation to return a map of size 1, but I also see some
convenience to directly returning the value (usually a single property). I will
take some time next week to work through some example queries and get a better
sense of how I feel on each option here.
There is one final item which I would like to see added to the proposal. I
think that all "variables" bound in the match query should be stored such that
they are later selectable. Essentially I think it's important to support
something like this:
g.match("MATCH (n1{name:'Cole'})-[]->(n2) RETURN
n1").where(...)...select(n2).out()...
The ability to select other bound variables later in the traversal should
greatly limit the number of times users are forced to return multiple items at
once, which reduces the amount of use cases where users will be forced to break
down maps in gremlin to complete their query.
Overall I think this would be a great change to gremlin. I look forward to
keeping this discussion going and ultimately seeing the changes land in
TinkerPop.
Thanks,
Cole
On 2025/08/22 15:46:10 Andrii Lomakin wrote:
> Good day.
>
> I propose new semantics for the match step in Gremlin, which we discussed
> briefly in the Discord chat. The current ideas listed partially summarize
> ideas suggested by several discussion participants.
>
> The current semantics of the match step are complex to optimize, so users
> do not use this step in practice, and DB vendors do not recommend using
> match step in queries.
>
> Instead, what is proposed is to provide a new match step based on
> declarative semantics.
>
> Signature of this step is quite simple: Travervsal<S, E> match(String
> matchQuery).
>
> Where matchQuery is a match statement written in declarative query language
> supported by the provider, I will use GQL as an example below.
>
> This step will require the language as a configuration parameter provided
> using with the step.
>
> So the simplest query will look like:
>
> g.match(“MATCH (person:Person)-[:knows]->(friend:Person)”).with(“language”,
> “GQL”)
>
> match step can accept query parameters, so if we provide a query like
> g.match(“MATCH
> (p:Person WHERE p.name = $personName)RETURN p.email”).with(“language”,
> “GQL”)
>
> we may use parameter bindings, but it will work only for interaction with
> Gremlin Server, so instead, I propose an additional modulator step:
> withParameter(String
> name, Object value)
>
> In such case final version will look like: g.match(“MATCH (p:Person WHERE
> p.name = $personName) RETURN p.email”).with(“language”,
> “GQL”).withParameter(“personName”, “Stephen”)
>
> Alongside the version of withParameter step that provides the name of the
> query parameter, a version with the following signature should also be
> provided: withParameter(int index, Object value) for query languages that
> support indexed parameters with/instead of named parameters.
>
> Because we already introduced one modulator step, it is reasonable to
> consider replacing it with step by more specific withQueryLanguage()
> modulator step that will allow us to add more expressiveness to the
> resulting queries.
>
> In such case final version will look like: g.match(“MATCH (p:Person WHERE
> p.name = $personName) RETURN
> p.email”).withQueryLanguage(“GQL”).withParameter(“personName”, “Stephen”)
>
> As for the scope of application of this step, I recommend making it behave
> exactly as it is implemented for the V() and E() steps. It could be added
> in the middle of GraphTraversal, but the execution result will be the same
> pattern matching execution applied to the whole graph stored in the
> database (not to the item filtered/transformed by the previous steps).
>
> It also means that match step will be added to the GraphTraversalSource.
>
> As for the format of the output of the match step, I would recommend the
> following:
>
> 1. If the match statement returns an Element instance, it is returned as
> is.
>
> 2. Otherwise, it should return any value that is allowed to be a property
> value in Element.
>
> 3. I would add an optional recommendation to return either Element or
> Map<String,
> ?> where the key of the map is the result a projection of the query result
> which in case of query g.match(“MATCH (p:Person WHERE p.name =
> $personName) RETURN
> p.email”).withQueryLanguage(“GQL”).withParameter(“personName”, “Stephen”)
>
> will look like {“p.email”: “[email protected]”}. Following this optional
> recommendation will, IMHO, improve user experience.
>
> This step should be restricted to executing only idempotent queries.
>
> I would also recommend adding versions of withParameter() that accept
> Traversal as a value of the parameters, namely:
> 1. withParameter(String name, TraversalSource value)
>
> 2. withParameter(int index, TraversalSource value)
>
>
>
> The current version of the match step should be deprecated and then removed.
>
> I want to thank Stephen Mallette, whose initial idea closely aligned with
> ours and who actively contributed to our discussions.
>
> I'm looking forward to your thoughts, observations, and any other feedback
> you may have.
>
> Best Regards,
> YouTrackDB development lead
> Andrii Lomakin
>
