feat(cognitarium): introduce query plan model

contracts/okp4-cognitarium/src/querier/plan.rs

@@ -0,0 +1,51 @@
+use crate::msg::{Prefix, SelectQuery, SimpleWhereCondition, TriplePattern, WhereCondition};
+use crate::state::{Object, Predicate, Subject};
+use std::collections::{BTreeMap, BTreeSet, HashMap};
+
+/// Represents a querying plan.
+pub struct QueryPlan {
+    /// References the ending node of the plan, when evaluated others nodes will be invoked in
+    /// cascade.
+    pub entrypoint: QueryNode,
+
+    /// Contains all the query variables, their index in this array are internally used as
+    /// identifiers.
+    pub variables: Vec<String>,
+}
+
+/// Represents a single part of the query plan processing. Each node is intended to provide a
+/// specific behavior given an evaluation context.
+pub enum QueryNode {
+    /// Match the triple pattern against the state. The triple elements can be either a variable or
+    /// a constant value, in the case of a variable it'll be either provided by the context of
+    /// previous evaluation or calculated and present in output.
+    TriplePattern {
+        subject: PatternValue<Subject>,
+        predicate: PatternValue<Predicate>,
+        object: PatternValue<Object>,
+    },
+
+    /// Join two nodes by applying the cartesian product of the nodes variables.
+    ///
+    /// This should be used when the nodes doesn't have variables in common, and can be seen as a
+    /// full join of disjoint datasets.  
+    CartesianProductJoin { left: Self, right: Self },
+
+    /// Join two nodes by using the variables values from the left node as replacement in the right
+    /// node.
+    ///
+    /// This results to an inner join, but the underlying processing stream the variables from the
+    /// left node to use them as right node values.
+    ForLoopJoin { left: Self, right: Self },
+
+    /// Skip the specified first elements from the child node.
+    Skip { child: Self, first: usize },
+
+    /// Limit to the specified first elements from the child node.
+    Limit { child: Self, first: usize },
+}
+
+pub enum PatternValue<V> {
+    Constant(V),
+    Variable(usize),
+}