[jira] [Closed] (TINKERPOP3-700) WhereStep should "MatchStep" and ConjunctionP should use the BudgetAlgorithm

["Marko A. Rodriguez (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/TINKERPOP3-700?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Marko A. Rodriguez closed TINKERPOP3-700.
-----------------------------------------
       Resolution: Fixed
    Fix Version/s: 3.0.0.GA
         Assignee: Marko A. Rodriguez

This is complete. The solution boiled down to the repeated {{"select(x)"}}-model proposed (ultimately what was written in Gremlin at the end of this ticket, but now rewritten in Java for efficiency). 

The idea of {{AnyP}} in this ticket can be used in a future {{MatchAlgorithm}} on semantically equivalent patterns. Will just store that in the old noggin' for now.

> WhereStep should "MatchStep" and ConjunctionP should use the BudgetAlgorithm
> ----------------------------------------------------------------------------
>
>                 Key: TINKERPOP3-700
>                 URL: https://issues.apache.org/jira/browse/TINKERPOP3-700
>             Project: TinkerPop 3
>          Issue Type: Improvement
>          Components: process
>            Reporter: Marko A. Rodriguez
>            Assignee: Marko A. Rodriguez
>             Fix For: 3.0.0.GA
>
>
> {code}
> g.V.as('a').where(a.knows.b
>                   a.knows.c
>                   b.knows.c)
> {code}
> The above can be written as an OrP of the form:
> {code}
> g.V.as('a').or(select(a).knows.b.select(b).knows.c.select(a).knows.where(eq(c)),
>                select(a).knows.c.select(a).knows.b.select(b).knows.where(eq(c)));
> {code}
> In essence, the where-statements are rewritten in terms of every possible permutation. When these permutations are put into an OrP (via or(traversals…)), then if any branch returns a result, then the original 'a' is emitted (as {{WhereStep}} is a {{FilterStep}}). If {{OrP}} is under the BudgetAlgorithm, then {{OrP}} will "thread between" its traversals until a value is yielded. *And given that all permutations are the same semantics -- if one fails, they all fail!*
> What is nice about this, is that arbitrary nesting comes "for free."
> {code}
> g.V.as('a').where(a.knows.b
>                   a.uncle.b
>                     and(a.worksFor.c
>                           b.worksFor.c))
> {code}
> This is rewritten as:
> {code}
> g.V.as('a').or(select(a).knows.b.or(select(a).worksFor.c.select(b).worksFor.where(eq(c)),select(b).worksFor.c.select(a).worksFor.where(eq(c))).select(a).uncle.where(eq(b)),
>                select(a).uncle.b.select(a).knows.where(eq(b)).or(select(a).worksFor.c.select(b).worksFor.where(eq(c)),select(b).worksFor.c.select(a).worksFor.where(eq(c))))
> {code}
> *IMPORTANT* This is not a "match" in the {{MatchStep}} sense as it doesn't return all permutations that bind, it only filters based on a single match.
> What is interesting about this approach:
>   1. The rewrite algorithm seems simple as its just concatenation given {{select()}}-projections and {{where(eq)}}-tails.
>   2. The cool thing about the rewrite in all possible permutations is that if any one {{FastNoSuchElementException}}, its booted from the {{ConjunctionP}} analysis.
>   3. {{ConjunctionP}} has the BudgetAlgorithm and thus can be used for ANY step that has conjunctions -- {{HasStep}}, {{IsStep}}, etc.
>   4. It uses the path data structure to maintain the variable bindings. {{WhereStep}} has no state! Its all about {{OrP}}. 
>   5. Given that the path data is the variable bindings, then this also works for OLAP as the traverser contains all the information it needs (no central location of analysis!)
> 	- However, you would only pick one permutation to do as `or()` does not exist in OLAP. 
> 	- and with one permutation, {{where().select()}} is then {{MatchStep}} which would then work in OLAP!
> 		- thus, Gremlin OLAP can rewrite {{match()}} to the {{where().select()}} form and TADA!
>   
> *IMPORTANT* 4 and 5 above are pretty insane consequences. And if any, we should at least use this realization to make {{match()}} work in OLAP.
> Next, realize that how {{where()}} should work is that if an {{as()}} is NOT in the path data structure, then its a variable bindings for rewrite. Moreover, if you don't provide a start {{as()}}, it is assumed to be the incoming object (currently how {{where()}} works). For example:
> {code}
> g.V.where(knows.b
>           knows.c
>           b.knows.c)
> {code}
> This is rewritten as:
> {code}
> g.V.or(x.select(x).knows.b.or(select(b).worksFor.where(eq(a)),select(x).worksFor.where(eq(b))).select(x).uncle.where(eq(b)),
>        x.select(x).uncle.b.select(x).knows.where(eq(b)).or(select(b).worksFor.where(eq(x)),select(x).worksFor.where(eq(b))))  
> {code}
> To be sure, the {{as('a').select('a')}} fragments can of course be optimized out to just {{as('a')}}.
>           



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