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Posted to commits@jena.apache.org by an...@apache.org on 2022/11/21 20:30:08 UTC
[jena-site] branch lateral-doc created (now 36d1de8af)
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at 36d1de8af Documentation for LATERAL
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new 36d1de8af Documentation for LATERAL
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[jena-site] 01/01: Documentation for LATERAL
Posted by an...@apache.org.
This is an automated email from the ASF dual-hosted git repository.
andy pushed a commit to branch lateral-doc
in repository https://gitbox.apache.org/repos/asf/jena-site.git
commit 36d1de8af8ced0f964b742815288d99f8e42d211
Author: Andy Seaborne <an...@apache.org>
AuthorDate: Mon Nov 21 20:29:57 2022 +0000
Documentation for LATERAL
---
source/documentation/query/__index.md | 1 +
source/documentation/query/lateral.md | 174 ++++++++++++++++++++++++++++++++++
2 files changed, 175 insertions(+)
diff --git a/source/documentation/query/__index.md b/source/documentation/query/__index.md
index bdb3da82b..7d9063935 100644
--- a/source/documentation/query/__index.md
+++ b/source/documentation/query/__index.md
@@ -69,6 +69,7 @@ Features of ARQ that are legal SPARQL syntax
Feature of ARQ that go beyond SPARQL syntax.
+- [LATERAL Join](lateral-join.html)
- [RDF-star](https://w3c.github.io/rdf-star/)
- Operators and functions
`[MOD](https://www.w3.org/TR/xpath-functions/#func-numeric-mod)`
diff --git a/source/documentation/query/lateral.md b/source/documentation/query/lateral.md
new file mode 100644
index 000000000..698fb1a2c
--- /dev/null
+++ b/source/documentation/query/lateral.md
@@ -0,0 +1,174 @@
+---
+title: ARQ - Lateral Join
+---
+
+Lateral joins using the keyword `LATERAL` were introduced in Apache jena 4.7.0.
+
+A `LATERAL` join is like a foreach loop, looping on the results from the
+left-hand side (LHS), the pattern before the `LATERAL` keyword, and executing
+the right-hand side (RHS) query pattern once for each row, with the variables
+from the RHS in-scope during each RHS evaluation.
+
+A regular join only executes the RHS once, and the variables from the LHS are
+only used for the join condition after evaluation of the left and right
+sub-patterns.
+
+Another way to think of a lateral join is as a `flatmap`.
+
+Examples:
+```
+## Get exactly one label for each subject with type `:T`
+SELECT * {
+ ?s rdf:type :T
+ LATERAL {
+ SELECT * { ?s rdfs:label ?label } LIMIT 1
+ }
+}
+```
+
+```
+## Get zero or one labels for each subject.
+SELECT * {
+ ?s ?p ?o
+ LATERAL { OPTIONAL { SELECT * ?s rdfs:label ?label } LIMIT 1}
+ }
+}
+```
+
+### Syntax
+
+The `LATERAL` keyword which takes the graph pattern so far in the group, from
+the `{` starting of the current block, and a `{ }` block afterwards.
+
+### Evaluation
+
+[Substituting variables](https://afs.github.io/substitute.html) from the LHS into the RHS (with the same restrictions), then executing the pattern, gives the evaluation of `LATERAL`
+
+### Variable Scopes
+
+In looping on the input, a lateral join makes the bindings of variables in the current row
+available to the right hand side pattern, settign their value from the top down.
+
+In SPARQL, it is possible to have variables of the same name which are not
+exposed within a sub-select. These are not lateral-joined to a variable of the
+same name from the LHS.
+
+This is not speciifc to lateral joins. In
+```
+SELECT * {
+ ?s rdf:type :T
+ { SELECT ?label { ?s rdfs:label ?label } }
+```
+the inner `?s` can be replaced by `?z` without changing the results because the
+inner `?s` is not joined to the outer `?s` but instead is hidden by the `SELECT ?label`.
+```
+SELECT * {
+ ?s rdf:type :T
+ { SELECT ?label { ?z rdfs:label ?label } }
+```
+
+The same rule applies to lateral joins.
+```
+SELECT * {
+ ?s rdf:type :T
+ LATERAL {
+ SELECT ?label { ?s rdfs:label ?label } LIMIT 1
+ }
+}
+```
+
+The inner `?s` in the `SELECT ?label` is not the outer `?s` because the `SELECT
+?label` does not pass out `?s`. As a sub-query the `?s` could be any name except
+`?label` for the same results.
+
+### Variable assignment
+
+There needs to be a new syntax restriction: there can no variable introduced by
+`AS` (`BIND`, or sub-query) or `VALUES` in-scope at the top level of the
+`LATERAL` RHS, that is the same name as any
+[in-scope](https://www.w3.org/TR/sparql11-query/#variableScope) variable from
+the LHS.
+
+Such a variable assignment would conflict with the variable being set in
+variables of the row being joined.
+
+```
+## ** Illegal **
+SELECT * {
+ ?s ?p ?o
+ LATERAL { BIND( 123 AS ?o) }
+}
+```
+
+See [SPARQL Grammar note 12](https://www.w3.org/TR/sparql11-query/#sparqlGrammar).
+
+In ARQ, [LET](https://www.w3.org/TR/sparql11-query/#variableScope) would work.
+`LET` for a variable that is bound acts like a filter.
+
+### Notes
+
+There is a similarity to filter `NOT EXISTS`/`EXISTS` expressed as the not-legal `FILTER ( ASK { pattern } )` where the variables of the row being filtered are available to "pattern". This is similar to ab SQL [correlated subquery](https://en.wikipedia.org/wiki/Correlated_subquery).
+
+## SPARQL Specification Additional
+
+### Syntax
+
+`LATERAL` is added to the SPARQL grammar at rule `[[56] GraphPatternNotTriples](https://www.w3.org/TR/sparql11-query/#rGraphPatternNotTriples)`. As a syntax form, it is similar to `OPTIONAL`.
+
+```
+[56] GraphPatternNotTriples ::= GroupOrUnionGraphPattern | OptionalGraphPattern | LateralGraphPattern | ...
+[57] OptionalGraphPattern ::= 'OPTIONAL' GroupGraphPattern
+[ ] LateralGraphPattern ::= 'LATERAL' GroupGraphPattern
+```
+
+### Algebra
+
+The new algebra is operator is `lateral` which takes two expressions
+
+```
+ SELECT * {
+ ?s ?p ?o
+ LATERAL
+ { ?a ?b ?c }
+}
+```
+is translated to:
+```
+ (lateral
+ (bgp (triple ?s ?p ?o))
+ (bgp (triple ?a ?b ?c)))
+```
+
+### Evaluation
+
+To evaluate `lateral`:
+
+* Evaluate the first argument (left-hand side from syntax) to get a multiset of solution mappings.
+* For each solution mapping ("row"),
+ inject variable bindings into the second argument
+ Evaluate this pattern
+ Add to results
+
+Outline:
+```
+Definition: Lateral
+
+Let Ω be a multiset of solution mappings. We define:
+
+Lateral(Ω, P) = { μ | union of Ω1 where
+ foreach μ1 in Ω:
+ pattern2 = inject(pattern, μ1)
+ Ω1 = eval(D(G), pattern2)
+ result Ω1
+ }
+```
+where `inject` is the [corrected `substitute`](https://afs.github.io/substitute.html) operation.
+
+An alternative style is to define Lateral more like "evaluate P such that μ is in-scope" in some
+way, rather than rely on `inject` which is a mechanism.
+
+```
+Definition: Evaluation of Lateral
+
+eval(D(G), Lateral(P1, P2) = Lateral(eval(D(G), P1), P2)
+```