Implement projection and querying. (#353) r=nalexander

* Add a failing test for EDN parsing '…'.
* Expose a SQLValueType trait to get value_type_tag values out of a ValueType.
* Add accessors to FindSpec.
* Implement querying.
* Implement rudimentary projection.
* Export mentat_db::new_connection.
* Export symbols from mentat.
* Add rudimentary end-to-end query tests.

Cargo.toml

@@ -48,11 +48,17 @@ path = "db"
 [dependencies.mentat_query]
 path = "query"
 
+[dependencies.mentat_query_algebrizer]
+path = "query-algebrizer"
+
 [dependencies.mentat_query_parser]
 path = "query-parser"
 
-[dependencies.mentat_query_algebrizer]
+[dependencies.mentat_query_projector]
-path = "query-algebrizer"
+path = "query-projector"
+
+[dependencies.mentat_query_sql]
+path = "query-sql"
 
 [dependencies.mentat_query_translator]
 path = "query-translator"

db/src/db.rs

@@ -331,6 +331,25 @@ pub fn ensure_current_version(conn: &mut rusqlite::Connection) -> Result<DB> {
     }
 }
 
+pub trait SQLValueType {
+    fn value_type_tag(&self) -> i32;
+}
+
+impl SQLValueType for ValueType {
+    fn value_type_tag(&self) -> i32 {
+        match *self {
+            ValueType::Ref =>      0,
+            ValueType::Boolean =>  1,
+            ValueType::Instant =>  4,
+            // SQLite distinguishes integral from decimal types, allowing long and double to share a tag.
+            ValueType::Long =>     5,
+            ValueType::Double =>   5,
+            ValueType::String =>  10,
+            ValueType::Keyword => 13,
+        }
+    }
+}
+
 pub trait TypedSQLValue {
     fn from_sql_value_pair(value: rusqlite::types::Value, value_type_tag: i32) -> Result<TypedValue>;
     fn to_sql_value_pair<'a>(&'a self) -> (ToSqlOutput<'a>, i32);

db/src/lib.rs

@@ -40,6 +40,12 @@ mod upsert_resolution;
 mod values;
 mod tx;
 
+pub use db::{
+    SQLValueType,
+    TypedSQLValue,
+    new_connection,
+};
+
 pub use tx::transact;
 pub use types::{
     DB,

query-algebrizer/src/cc.rs

@@ -193,12 +193,12 @@ pub struct ConjoiningClauses {
     pub wheres: Vec<ColumnConstraint>,
 
     /// A map from var to qualified columns. Used to project.
-    bindings: BTreeMap<Variable, Vec<QualifiedAlias>>,
+    pub bindings: BTreeMap<Variable, Vec<QualifiedAlias>>,
 
     /// A map from var to type. Whenever a var maps unambiguously to two different types, it cannot
     /// yield results, so we don't represent that case here. If a var isn't present in the map, it
     /// means that its type is not known in advance.
-    known_types: BTreeMap<Variable, ValueType>,
+    pub known_types: BTreeMap<Variable, ValueType>,
 
     /// A mapping, similar to `bindings`, but used to pull type tags out of the store at runtime.
     /// If a var isn't present in `known_types`, it should be present here.

query-algebrizer/src/lib.rs

@@ -27,9 +27,9 @@ use mentat_query::{
 #[allow(dead_code)]
 pub struct AlgebraicQuery {
     default_source: SrcVar,
-    find_spec: FindSpec,
+    pub find_spec: FindSpec,
     has_aggregates: bool,
-    limit: Option<i64>,
+    pub limit: Option<i64>,
     pub cc: cc::ConjoiningClauses,
 }
 

query-projector/Cargo.toml

@@ -0,0 +1,35 @@
+[package]
+name = "mentat_query_projector"
+version = "0.0.1"
+workspace = ".."
+
+[dependencies]
+error-chain = "0.9.0"
+
+[dependencies.rusqlite]
+version = "0.9.5"
+# System sqlite might be very old.
+features = ["bundled"]
+
+[dependencies.mentat_core]
+path = "../core"
+
+[dependencies.mentat_db]
+path = "../db"
+
+[dependencies.mentat_sql]
+path = "../sql"
+
+[dependencies.mentat_query]
+path = "../query"
+
+[dependencies.mentat_query_algebrizer]
+path = "../query-algebrizer"
+
+# Only for tests.
+[dev-dependencies.mentat_query_parser]
+path = "../query-parser"
+
+[dependencies.mentat_query_sql]
+path = "../query-sql"
+

query-projector/README.md

@@ -0,0 +1,6 @@
+This module handles the derivation from an algebrized query of two things:
+
+- A SQL projection: a mapping from columns mentioned in the body of the query to columns in the output.
+- A Datalog projection: a function that consumes rows of the appropriate shape (as defined by the SQL projection) to yield one of the four kinds of Datalog query result.
+
+These two must naturally coordinate, and so they are both produced here.

query-projector/src/lib.rs

@@ -0,0 +1,423 @@
+// Copyright 2016 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+#[macro_use]
+extern crate error_chain;
+extern crate rusqlite;
+
+extern crate mentat_core;
+extern crate mentat_db;                 // For value conversion.
+extern crate mentat_query;
+extern crate mentat_query_algebrizer;
+extern crate mentat_query_sql;
+extern crate mentat_sql;
+
+use std::iter;
+
+use rusqlite::{
+    Row,
+    Rows,
+};
+
+use mentat_core::{
+    TypedValue,
+};
+
+use mentat_db::{
+    SQLValueType,
+    TypedSQLValue,
+};
+
+use mentat_query::{
+    Element,
+    FindSpec,
+    PlainSymbol,
+    Variable,
+};
+
+use mentat_query_algebrizer::{
+    AlgebraicQuery,
+    DatomsColumn,
+    QualifiedAlias,
+    /*
+    ConjoiningClauses,
+    DatomsTable,
+    SourceAlias,
+    */
+};
+
+use mentat_query_sql::{
+    ColumnOrExpression,
+    /*
+    Constraint,
+    FromClause,
+    */
+    Name,
+    Projection,
+    ProjectedColumn,
+    /*
+    SelectQuery,
+    TableList,
+    */
+};
+
+error_chain! {
+    types {
+        Error, ErrorKind, ResultExt, Result;
+    }
+
+    foreign_links {
+        Rusqlite(rusqlite::Error);
+    }
+
+    links {
+        DbError(mentat_db::Error, mentat_db::ErrorKind);
+    }
+}
+
+#[derive(Debug)]
+pub enum QueryResults {
+    Scalar(Option<TypedValue>),
+    Tuple(Option<Vec<TypedValue>>),
+    Coll(Vec<TypedValue>),
+    Rel(Vec<Vec<TypedValue>>),
+}
+
+impl QueryResults {
+    pub fn len(&self) -> usize {
+        use QueryResults::*;
+        match self {
+            &Scalar(ref o) => if o.is_some() { 1 } else { 0 },
+            &Tuple(ref o)  => if o.is_some() { 1 } else { 0 },
+            &Coll(ref v)   => v.len(),
+            &Rel(ref v)    => v.len(),
+        }
+    }
+}
+
+type Index = i32;            // See rusqlite::RowIndex.
+type ValueTypeTag = i32;
+enum TypedIndex {
+    Known(Index, ValueTypeTag),
+    Unknown(Index, Index),
+}
+
+impl TypedIndex {
+    /// Look up this index and type(index) pair in the provided row.
+    /// This function will panic if:
+    ///
+    /// - This is an `Unknown` and the retrieved type code isn't an i32.
+    /// - If the retrieved value can't be coerced to a rusqlite `Value`.
+    /// - Either index is out of bounds.
+    ///
+    /// Because we construct our SQL projection list, the code that stored the data, and this
+    /// consumer, a panic here implies that we have a bad bug — we put data of a very wrong type in
+    /// a row, and thus can't coerce to Value, we're retrieving from the wrong place, or our
+    /// generated SQL is junk.
+    ///
+    /// This function will return a runtime error if the type code is unknown, or the value is
+    /// otherwise not convertible by the DB layer.
+    fn lookup<'a, 'stmt>(&self, row: &Row<'a, 'stmt>) -> Result<TypedValue> {
+        use TypedIndex::*;
+
+        match self {
+            &Known(value_index, value_type) => {
+                let v: rusqlite::types::Value = row.get(value_index);
+                TypedValue::from_sql_value_pair(v, value_type).map_err(|e| e.into())
+            },
+            &Unknown(value_index, type_index) => {
+                let v: rusqlite::types::Value = row.get(value_index);
+                let value_type_tag: i32 = row.get(type_index);
+                TypedValue::from_sql_value_pair(v, value_type_tag).map_err(|e| e.into())
+            },
+        }
+    }
+}
+
+fn column_name(var: &Variable) -> Name {
+    let &Variable(PlainSymbol(ref s)) = var;
+    s.clone()
+}
+
+fn value_type_tag_name(var: &Variable) -> Name {
+    let &Variable(PlainSymbol(ref s)) = var;
+    format!("{}_value_type_tag", s)
+}
+
+/// Walk an iterator of `Element`s, collecting projector templates and columns.
+///
+/// Returns a pair: the SQL projection (which should always be a `Projection::Columns`)
+/// and a `Vec` of `TypedIndex` 'keys' to use when looking up values.
+///
+/// Callers must ensure that every `Element` is distinct -- a query like
+///
+/// ```edn
+/// [:find ?x ?x :where [?x _ _]]
+/// ```
+///
+/// should fail to parse. See #358.
+fn project_elements<'a, I: IntoIterator<Item = &'a Element>>(
+    count: usize,
+    elements: I,
+    query: &AlgebraicQuery) -> (Projection, Vec<TypedIndex>) {
+
+    let mut cols = Vec::with_capacity(count);
+    let mut i: i32 = 0;
+    let mut templates = vec![];
+
+    for e in elements {
+        match e {
+            // Each time we come across a variable, we push a SQL column
+            // into the SQL projection, aliased to the name of the variable,
+            // and we push an annotated index into the projector.
+            &Element::Variable(ref var) => {
+                // Every variable should be bound by the top-level CC to at least
+                // one column in the query. If that constraint is violated it's a
+                // bug in our code, so it's appropriate to panic here.
+                let columns = query.cc
+                                   .bindings
+                                   .get(var)
+                                   .expect("Every variable has a binding");
+
+                let qa = columns[0].clone();
+                let name = column_name(var);
+
+                if let Some(t) = query.cc.known_types.get(var) {
+                    cols.push(ProjectedColumn(ColumnOrExpression::Column(qa), name));
+                    let tag = t.value_type_tag();
+                    templates.push(TypedIndex::Known(i, tag));
+                    i += 1;     // We used one SQL column.
+                } else {
+                    let table = qa.0.clone();
+                    cols.push(ProjectedColumn(ColumnOrExpression::Column(qa), name));
+                    templates.push(TypedIndex::Unknown(i, i + 1));
+                    i += 2;     // We used two SQL columns.
+
+                    // Also project the type from the SQL query.
+                    let type_name = value_type_tag_name(var);
+                    let type_qa = QualifiedAlias(table, DatomsColumn::ValueTypeTag);
+                    cols.push(ProjectedColumn(ColumnOrExpression::Column(type_qa), type_name));
+                }
+            }
+        }
+    }
+
+    (Projection::Columns(cols), templates)
+}
+
+pub trait Projector {
+    fn project<'stmt>(&self, rows: Rows<'stmt>) -> Result<QueryResults>;
+}
+
+struct ScalarProjector {
+    template: TypedIndex,
+}
+
+impl ScalarProjector {
+    fn with_template(template: TypedIndex) -> ScalarProjector {
+        ScalarProjector {
+            template: template,
+        }
+    }
+
+    fn combine(sql: Projection, mut templates: Vec<TypedIndex>) -> CombinedProjection {
+        let template = templates.pop().expect("Expected a single template");
+        CombinedProjection {
+            sql_projection: sql,
+            datalog_projector: Box::new(ScalarProjector::with_template(template)),
+        }
+    }
+}
+
+impl Projector for ScalarProjector {
+    fn project<'stmt>(&self, mut rows: Rows<'stmt>) -> Result<QueryResults> {
+        if let Some(r) = rows.next() {
+            let row = r?;
+            let binding = self.template.lookup(&row)?;
+            Ok(QueryResults::Scalar(Some(binding)))
+        } else {
+            Ok(QueryResults::Scalar(None))
+        }
+    }
+}
+
+/// A tuple projector produces a single vector. It's the single-result version of rel.
+struct TupleProjector {
+    len: usize,
+    templates: Vec<TypedIndex>,
+}
+
+impl TupleProjector {
+    fn with_templates(len: usize, templates: Vec<TypedIndex>) -> TupleProjector {
+        TupleProjector {
+            len: len,
+            templates: templates,
+        }
+    }
+
+    // This is exactly the same as for rel.
+    fn collect_bindings<'a, 'stmt>(&self, row: Row<'a, 'stmt>) -> Result<Vec<TypedValue>> {
+        assert_eq!(row.column_count(), self.len as i32);
+        self.templates
+            .iter()
+            .map(|ti| ti.lookup(&row))
+            .collect::<Result<Vec<TypedValue>>>()
+    }
+
+    fn combine(column_count: usize, sql: Projection, templates: Vec<TypedIndex>) -> CombinedProjection {
+        let p = TupleProjector::with_templates(column_count, templates);
+        CombinedProjection {
+            sql_projection: sql,
+            datalog_projector: Box::new(p),
+        }
+    }
+}
+
+impl Projector for TupleProjector {
+    fn project<'stmt>(&self, mut rows: Rows<'stmt>) -> Result<QueryResults> {
+        if let Some(r) = rows.next() {
+            let row = r?;
+            let bindings = self.collect_bindings(row)?;
+            Ok(QueryResults::Tuple(Some(bindings)))
+        } else {
+            Ok(QueryResults::Tuple(None))
+        }
+    }
+}
+
+/// A rel projector produces a vector of vectors.
+/// Each inner vector is the same size, and sourced from the same columns.
+/// One inner vector is produced per `Row`.
+/// Each column in the inner vector is the result of taking one or two columns from
+/// the `Row`: one for the value and optionally one for the type tag.
+struct RelProjector {
+    len: usize,
+    templates: Vec<TypedIndex>,
+}
+
+impl RelProjector {
+    fn with_templates(len: usize, templates: Vec<TypedIndex>) -> RelProjector {
+        RelProjector {
+            len: len,
+            templates: templates,
+        }
+    }
+
+    fn collect_bindings<'a, 'stmt>(&self, row: Row<'a, 'stmt>) -> Result<Vec<TypedValue>> {
+        assert_eq!(row.column_count(), self.len as i32);
+        self.templates
+            .iter()
+            .map(|ti| ti.lookup(&row))
+            .collect::<Result<Vec<TypedValue>>>()
+    }
+
+    fn combine(column_count: usize, sql: Projection, templates: Vec<TypedIndex>) -> CombinedProjection {
+        let p = RelProjector::with_templates(column_count, templates);
+        CombinedProjection {
+            sql_projection: sql,
+            datalog_projector: Box::new(p),
+        }
+    }
+}
+
+impl Projector for RelProjector {
+    fn project<'stmt>(&self, mut rows: Rows<'stmt>) -> Result<QueryResults> {
+        let mut out: Vec<Vec<TypedValue>> = vec![];
+        while let Some(r) = rows.next() {
+            let row = r?;
+            let bindings = self.collect_bindings(row)?;
+            out.push(bindings);
+        }
+        Ok(QueryResults::Rel(out))
+    }
+}
+
+/// A coll projector produces a vector of values.
+/// Each value is sourced from the same column.
+struct CollProjector {
+    template: TypedIndex,
+}
+
+impl CollProjector {
+    fn with_template(template: TypedIndex) -> CollProjector {
+        CollProjector {
+            template: template,
+        }
+    }
+
+    fn combine(sql: Projection, mut templates: Vec<TypedIndex>) -> CombinedProjection {
+        let template = templates.pop().expect("Expected a single template");
+        CombinedProjection {
+            sql_projection: sql,
+            datalog_projector: Box::new(CollProjector::with_template(template)),
+        }
+    }
+}
+
+impl Projector for CollProjector {
+    fn project<'stmt>(&self, mut rows: Rows<'stmt>) -> Result<QueryResults> {
+        let mut out: Vec<TypedValue> = vec![];
+        while let Some(r) = rows.next() {
+            let row = r?;
+            let binding = self.template.lookup(&row)?;
+            out.push(binding);
+        }
+        Ok(QueryResults::Coll(out))
+    }
+}
+
+/// Combines the two things you need to turn a query into SQL and turn its results into
+/// `QueryResults`.
+pub struct CombinedProjection {
+    /// A SQL projection, mapping columns mentioned in the body of the query to columns in the
+    /// output.
+    pub sql_projection: Projection,
+
+    /// A Datalog projection. This consumes rows of the appropriate shape (as defined by
+    /// the SQL projection) to yield one of the four kinds of Datalog query result.
+    pub datalog_projector: Box<Projector>,
+}
+
+/// Compute a suitable SQL projection for an algebrized query.
+/// This takes into account a number of things:
+/// - The variable list in the find spec.
+/// - The presence of any aggregate operations in the find spec. TODO: for now we only handle
+///   simple variables
+/// - The bindings established by the topmost CC.
+/// - The types known at algebrizing time.
+/// - The types extracted from the store for unknown attributes.
+pub fn query_projection(query: &AlgebraicQuery) -> CombinedProjection {
+    use self::FindSpec::*;
+
+    match query.find_spec {
+        FindColl(ref element) => {
+            let (cols, templates) = project_elements(1, iter::once(element), query);
+            CollProjector::combine(cols, templates)
+        },
+
+        FindScalar(ref element) => {
+            let (cols, templates) = project_elements(1, iter::once(element), query);
+            ScalarProjector::combine(cols, templates)
+        },
+
+        FindRel(ref elements) => {
+            let column_count = query.find_spec.expected_column_count();
+            let (cols, templates) = project_elements(column_count, elements, query);
+            RelProjector::combine(column_count, cols, templates)
+        },
+
+        FindTuple(ref elements) => {
+            let column_count = query.find_spec.expected_column_count();
+            let (cols, templates) = project_elements(column_count, elements, query);
+            TupleProjector::combine(column_count, cols, templates)
+        },
+    }
+}
+

query-sql/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "mentat_query_sql"
+version = "0.0.1"
+workspace = ".."
+
+[dependencies]
+[dependencies.mentat_core]
+path = "../core"
+
+[dependencies.mentat_sql]
+path = "../sql"
+
+[dependencies.mentat_query]
+path = "../query"
+
+[dependencies.mentat_query_algebrizer]
+path = "../query-algebrizer"
+
+# Only for tests.
+[dev-dependencies.mentat_query_parser]
+path = "../query-parser"

query-sql/src/lib.rs

@@ -8,7 +8,10 @@
 // CONDITIONS OF ANY KIND, either express or implied. See the License for the
 // specific language governing permissions and limitations under the License.
 
-#![allow(dead_code, unused_imports)]
+extern crate mentat_core;
+extern crate mentat_query;
+extern crate mentat_query_algebrizer;
+extern crate mentat_sql;
 
 use mentat_core::{
     Entid,
@@ -16,16 +19,11 @@ use mentat_core::{
 };
 
 use mentat_query_algebrizer::{
-    AlgebraicQuery,
-    ConjoiningClauses,
     DatomsColumn,
-    DatomsTable,
     QualifiedAlias,
     SourceAlias,
 };
 
-use mentat_sql;
-
 use mentat_sql::{
     BuildQueryResult,
     QueryBuilder,
@@ -80,6 +78,7 @@ impl Constraint {
     }
 }
 
+#[allow(dead_code)]
 enum JoinOp {
     Inner,
 }
@@ -94,6 +93,7 @@ pub struct Join {
     // TODO: constraints (ON, USING).
 }
 
+#[allow(dead_code)]
 enum TableOrSubquery {
     Table(SourceAlias),
     // TODO: Subquery.
@@ -278,6 +278,7 @@ impl SelectQuery {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use mentat_query_algebrizer::DatomsTable;
 
     #[test]
     fn test_end_to_end() {

query-translator/Cargo.toml

@@ -19,3 +19,9 @@ path = "../query-algebrizer"
 # Only for tests.
 [dev-dependencies.mentat_query_parser]
 path = "../query-parser"
+
+[dependencies.mentat_query_projector]
+path = "../query-projector"
+
+[dependencies.mentat_query_sql]
+path = "../query-sql"

query-translator/src/lib.rs

@@ -11,16 +11,18 @@
 extern crate mentat_core;
 extern crate mentat_query;
 extern crate mentat_query_algebrizer;
+extern crate mentat_query_projector;
+extern crate mentat_query_sql;
 extern crate mentat_sql;
 
 mod translate;
-mod types;
 
-pub use types::{
+pub use mentat_query_sql::{
     Projection,
 };
 
 pub use translate::{
     cc_to_exists,
     cc_to_select,
+    query_to_select,
 };

query-translator/src/translate.rs

@@ -10,6 +10,13 @@
 
 #![allow(dead_code, unused_imports)]
 
+use mentat_query::{
+    Element,
+    FindSpec,
+    PlainSymbol,
+    Variable,
+};
+
 use mentat_query_algebrizer::{
     AlgebraicQuery,
     ColumnConstraint,
@@ -20,11 +27,19 @@ use mentat_query_algebrizer::{
     SourceAlias,
 };
 
-use types::{
+use mentat_query_projector::{
+    CombinedProjection,
+    Projector,
+    query_projection,
+};
+
+use mentat_query_sql::{
     ColumnOrExpression,
     Constraint,
     FromClause,
+    Name,
     Projection,
+    ProjectedColumn,
     SelectQuery,
     TableList,
 };
@@ -57,10 +72,12 @@ impl ToConstraint for ColumnConstraint {
     }
 }
 
+pub struct CombinedSelectQuery {
+    pub query: SelectQuery,
+    pub projector: Box<Projector>,
+}
 
-/// Consume a provided `ConjoiningClauses` to yield a new
+fn cc_to_select_query(projection: Projection, cc: ConjoiningClauses) -> SelectQuery {
-/// `SelectQuery`. A projection list must also be provided.
-pub fn cc_to_select(projection: Projection, cc: ConjoiningClauses) -> SelectQuery {
     SelectQuery {
         projection: projection,
         from: FromClause::TableList(TableList(cc.from)),
@@ -71,6 +88,20 @@ pub fn cc_to_select(projection: Projection, cc: ConjoiningClauses) -> SelectQuer
     }
 }
 
-pub fn cc_to_exists(cc: ConjoiningClauses) -> SelectQuery {
+/// Consume a provided `ConjoiningClauses` to yield a new
-    cc_to_select(Projection::One, cc)
+/// `SelectQuery`. A projection list must also be provided.
+pub fn cc_to_select(projection: CombinedProjection, cc: ConjoiningClauses) -> CombinedSelectQuery {
+    let CombinedProjection { sql_projection, datalog_projector } = projection;
+    CombinedSelectQuery {
+        query: cc_to_select_query(sql_projection, cc),
+        projector: datalog_projector,
+    }
+}
+
+pub fn query_to_select(query: AlgebraicQuery) -> CombinedSelectQuery {
+    cc_to_select(query_projection(&query), query.cc)
+}
+
+pub fn cc_to_exists(cc: ConjoiningClauses) -> SelectQuery {
+    cc_to_select_query(Projection::One, cc)
 }

query-translator/tests/translate.rs

@@ -27,7 +27,7 @@ use mentat_core::{
 use mentat_query_parser::parse_find_string;
 use mentat_query_algebrizer::algebrize;
 use mentat_query_translator::{
-    cc_to_exists,
+    query_to_select,
 };
 
 use mentat_sql::SQLQuery;
@@ -42,7 +42,26 @@ fn add_attribute(schema: &mut Schema, e: Entid, a: Attribute) {
 }
 
 #[test]
-fn test_exists() {
+#[should_panic(expected = "parse failed")]
+fn test_coll() {
+    let mut schema = Schema::default();
+    associate_ident(&mut schema, NamespacedKeyword::new("foo", "bar"), 99);
+    add_attribute(&mut schema, 99, Attribute {
+        value_type: ValueType::String,
+        ..Default::default()
+    });
+
+    let input = r#"[:find [?x ...] :where [?x :foo/bar "yyy"]]"#;
+    let parsed = parse_find_string(input).expect("parse failed");
+    let algebrized = algebrize(&schema, parsed);
+    let select = query_to_select(algebrized);
+    let SQLQuery { sql, args } = select.query.to_sql_query().unwrap();
+    assert_eq!(sql, "SELECT `datoms00`.e AS `?x` FROM `datoms` AS `datoms00` WHERE `datoms00`.a = 99 AND `datoms00`.v = $v0");
+    assert_eq!(args, vec![("$v0".to_string(), "yyy".to_string())]);
+}
+
+#[test]
+fn test_rel() {
     let mut schema = Schema::default();
     associate_ident(&mut schema, NamespacedKeyword::new("foo", "bar"), 99);
     add_attribute(&mut schema, 99, Attribute {
@@ -51,10 +70,10 @@ fn test_exists() {
     });
 
     let input = r#"[:find ?x :where [?x :foo/bar "yyy"]]"#;
-    let parsed = parse_find_string(input).unwrap();
+    let parsed = parse_find_string(input).expect("parse failed");
     let algebrized = algebrize(&schema, parsed);
-    let select = cc_to_exists(algebrized.cc);
+    let select = query_to_select(algebrized);
-    let SQLQuery { sql, args } = select.to_sql_query().unwrap();
+    let SQLQuery { sql, args } = select.query.to_sql_query().unwrap();
-    assert_eq!(sql, "SELECT 1 FROM `datoms` AS `datoms00` WHERE `datoms00`.a = 99 AND `datoms00`.v = $v0");
+    assert_eq!(sql, "SELECT `datoms00`.e AS `?x` FROM `datoms` AS `datoms00` WHERE `datoms00`.a = 99 AND `datoms00`.v = $v0");
     assert_eq!(args, vec![("$v0".to_string(), "yyy".to_string())]);
 }

query/src/lib.rs

@@ -340,15 +340,27 @@ pub enum FindSpec {
 }
 
 /// Returns true if the provided `FindSpec` returns at most one result.
-pub fn is_unit_limited(spec: &FindSpec) -> bool {
+impl FindSpec {
-    match spec {
+    pub fn is_unit_limited(&self) -> bool {
-        &FindSpec::FindScalar(..) => true,
+        use FindSpec::*;
-        &FindSpec::FindTuple(..)  => true,
+        match self {
-        &FindSpec::FindRel(..)    => false,
+            &FindScalar(..) => true,
-        &FindSpec::FindColl(..)   => false,
+            &FindTuple(..)  => true,
+            &FindRel(..)    => false,
+            &FindColl(..)   => false,
         }
     }
 
+    pub fn expected_column_count(&self) -> usize {
+        use FindSpec::*;
+        match self {
+            &FindScalar(..) => 1,
+            &FindColl(..)   => 1,
+            &FindTuple(ref elems) | &FindRel(ref elems) => elems.len(),
+        }
+    }
+
+
     /// Returns true if the provided `FindSpec` cares about distinct results.
     ///
     /// I use the words "cares about" because find is generally defined in terms of producing distinct
@@ -367,8 +379,9 @@ pub fn is_unit_limited(spec: &FindSpec) -> bool {
     /// sampling projections we might not need to do it at the SQL level because we're consuming into
     /// a dupe-eliminating data structure like a Set, or we know that a particular query cannot produce
     /// duplicate results.
-pub fn requires_distinct(spec: &FindSpec) -> bool {
+    pub fn requires_distinct(&self) -> bool {
-    return !is_unit_limited(spec);
+        !self.is_unit_limited()
+    }
 }
 
 // Note that the "implicit blank" rule applies.

src/errors.rs

@@ -15,6 +15,7 @@ use rusqlite;
 use edn;
 use mentat_db;
 use mentat_query_parser;
+use mentat_query_projector;
 use mentat_sql;
 use mentat_tx_parser;
 
@@ -31,6 +32,7 @@ error_chain! {
     links {
         DbError(mentat_db::Error, mentat_db::ErrorKind);
         QueryParseError(mentat_query_parser::Error, mentat_query_parser::ErrorKind);
+        ProjectorError(mentat_query_projector::Error, mentat_query_projector::ErrorKind);
         SqlError(mentat_sql::Error, mentat_sql::ErrorKind);
         TxParseError(mentat_tx_parser::Error, mentat_tx_parser::ErrorKind);
     }

src/lib.rs

@@ -25,6 +25,7 @@ extern crate mentat_db;
 extern crate mentat_query;
 extern crate mentat_query_algebrizer;
 extern crate mentat_query_parser;
+extern crate mentat_query_projector;
 extern crate mentat_query_translator;
 extern crate mentat_sql;
 extern crate mentat_tx_parser;
@@ -46,6 +47,17 @@ pub fn get_connection() -> Connection {
     return Connection::open_in_memory().unwrap();
 }
 
+pub use mentat_db::{
+    new_connection,
+};
+
+pub use query::{
+    NamespacedKeyword,
+    PlainSymbol,
+    QueryResults,
+    q_once,
+};
+
 #[cfg(test)]
 mod tests {
     use edn::symbols::Keyword;

src/query.rs

@@ -10,6 +10,9 @@
 
 use std::collections::HashMap;
 
+use rusqlite;
+use rusqlite::types::ToSql;
+
 use mentat_core::{
     Schema,
     TypedValue,
@@ -17,6 +20,11 @@ use mentat_core::{
 
 use mentat_query_algebrizer::algebrize;
 
+pub use mentat_query::{
+    NamespacedKeyword,
+    PlainSymbol,
+};
+
 use mentat_query_parser::{
     parse_find_string,
 };
@@ -26,21 +34,15 @@ use mentat_sql::{
 };
 
 use mentat_query_translator::{
-    cc_to_select,
+    query_to_select,
-    Projection,
+};
+
+pub use mentat_query_projector::{
+    QueryResults,
 };
 
 use errors::Result;
 
-use rusqlite;
-
-pub enum QueryResults {
-    Scalar(Option<TypedValue>),
-    Tuple(Vec<TypedValue>),
-    Coll(Vec<TypedValue>),
-    Rel(Vec<Vec<TypedValue>>),
-}
-
 pub type QueryExecutionResult = Result<QueryResults>;
 
 /// Take an EDN query string, a reference to a open SQLite connection, a Mentat DB, and an optional
@@ -59,21 +61,22 @@ pub fn q_once<'sqlite, 'schema, 'query>
     // TODO: validate inputs.
     let parsed = parse_find_string(query)?;
     let algebrized = algebrize(schema, parsed);
-    let projection = Projection::Star;
+    let select = query_to_select(algebrized);
-    let select = cc_to_select(projection, algebrized.cc);
+    let SQLQuery { sql, args } = select.query.to_sql_query()?;
-    let SQLQuery { sql, args } = select.to_sql_query()?;
 
-    /*
     let mut statement = sqlite.prepare(sql.as_str())?;
 
-    let mut rows = if args.is_empty() {
+    let rows = if args.is_empty() {
         statement.query(&[])?
     } else {
-        statement.query_named(args.map(|(k, v)| (k.as_str(), &v)))?
+        let refs: Vec<(&str, &ToSql)> =
+            args.iter()
+                .map(|&(ref k, ref v)| (k.as_str(), v as &ToSql))
+                .collect();
+        statement.query_named(refs.as_slice())?
     };
-    */
 
-
+    select.projector
-
+          .project(rows)
-    Ok(QueryResults::Scalar(Some(TypedValue::Boolean(true))))
+          .map_err(|e| e.into())
 }

tests/query.rs

@@ -0,0 +1,142 @@
+// Copyright 2016 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+extern crate time;
+
+extern crate mentat;
+extern crate mentat_core;
+extern crate mentat_db;
+
+use mentat_core::{
+    TypedValue,
+    ValueType,
+};
+
+use mentat::{
+    NamespacedKeyword,
+    QueryResults,
+    new_connection,
+    q_once,
+};
+
+#[test]
+fn test_rel() {
+    let mut c = new_connection("").expect("Couldn't open conn.");
+    let db = mentat_db::db::ensure_current_version(&mut c).expect("Couldn't open DB.");
+
+    // Rel.
+    let start = time::PreciseTime::now();
+    let results = q_once(&c, &db.schema,
+                         "[:find ?x ?ident :where [?x :db/ident ?ident]]", None)
+        .expect("Query failed");
+    let end = time::PreciseTime::now();
+
+    // This will need to change each time we add a default ident.
+    assert_eq!(37, results.len());
+
+    // Every row is a pair of a Ref and a Keyword.
+    if let QueryResults::Rel(ref rel) = results {
+        for r in rel {
+            assert_eq!(r.len(), 2);
+            assert!(r[0].matches_type(ValueType::Ref));
+            assert!(r[1].matches_type(ValueType::Keyword));
+        }
+    } else {
+        panic!("Expected rel.");
+    }
+
+    println!("{:?}", results);
+    println!("Rel took {}µs", start.to(end).num_microseconds().unwrap());
+}
+
+#[test]
+fn test_failing_scalar() {
+    let mut c = new_connection("").expect("Couldn't open conn.");
+    let db = mentat_db::db::ensure_current_version(&mut c).expect("Couldn't open DB.");
+
+    // Scalar that fails.
+    let start = time::PreciseTime::now();
+    let results = q_once(&c, &db.schema,
+                         "[:find ?x . :where [?x :db/fulltext true]]", None)
+        .expect("Query failed");
+    let end = time::PreciseTime::now();
+
+    assert_eq!(0, results.len());
+
+    if let QueryResults::Scalar(None) = results {
+    } else {
+        panic!("Expected failed scalar.");
+    }
+
+    println!("Failing scalar took {}µs", start.to(end).num_microseconds().unwrap());
+}
+
+#[test]
+fn test_scalar() {
+    let mut c = new_connection("").expect("Couldn't open conn.");
+    let db = mentat_db::db::ensure_current_version(&mut c).expect("Couldn't open DB.");
+
+    // Scalar that succeeds.
+    let start = time::PreciseTime::now();
+    let results = q_once(&c, &db.schema,
+                         "[:find ?ident . :where [24 :db/ident ?ident]]", None)
+        .expect("Query failed");
+    let end = time::PreciseTime::now();
+
+    assert_eq!(1, results.len());
+
+    if let QueryResults::Scalar(Some(TypedValue::Keyword(ref kw))) = results {
+        // Should be '24'.
+        assert_eq!(&NamespacedKeyword::new("db.type", "keyword"), kw);
+        assert_eq!(24,
+                   db.schema.get_entid(kw).unwrap());
+    } else {
+        panic!("Expected scalar.");
+    }
+
+    println!("{:?}", results);
+    println!("Scalar took {}µs", start.to(end).num_microseconds().unwrap());
+}
+
+#[test]
+fn test_tuple() {
+    let mut c = new_connection("").expect("Couldn't open conn.");
+    let db = mentat_db::db::ensure_current_version(&mut c).expect("Couldn't open DB.");
+
+    // Tuple.
+    let start = time::PreciseTime::now();
+    let results = q_once(&c, &db.schema,
+                         "[:find [?index ?cardinality]
+                           :where [:db/txInstant :db/index ?index]
+                                  [:db/txInstant :db/cardinality ?cardinality]]",
+                         None)
+        .expect("Query failed");
+    let end = time::PreciseTime::now();
+
+    assert_eq!(1, results.len());
+
+    if let QueryResults::Tuple(Some(ref tuple)) = results {
+        let cardinality_one = NamespacedKeyword::new("db.cardinality", "one");
+        assert_eq!(tuple.len(), 2);
+        assert_eq!(tuple[0], TypedValue::Boolean(true));
+        assert_eq!(tuple[1], TypedValue::Ref(db.schema.get_entid(&cardinality_one).unwrap()));
+    } else {
+        panic!("Expected tuple.");
+    }
+
+    println!("{:?}", results);
+    println!("Tuple took {}µs", start.to(end).num_microseconds().unwrap());
+}
+
+#[test]
+fn test_coll() {
+    // We can't test Coll yet, because the EDN parser is incomplete.
+}
+