feat(query): Fetch query response in JSON format.

Allows row deserialization into a `T: Deserialize`,
which eliminates the limitations of `Query::fetch`:

* when the table schema is not known: `SELECT * from ?`
* when the table schema is not specified: `DESCRIBE TABLE ?`
* when we read less columns than we select

src/query.rs

@@ -3,6 +3,8 @@ use serde::{Deserialize, Serialize};
 use std::fmt::Display;
 use url::Url;
 
+#[cfg(feature = "watch")]
+use crate::watch;
 use crate::{
     error::{Error, Result},
     headers::with_request_headers,
@@ -90,6 +92,60 @@ impl Query {
         Ok(RowCursor::new(response))
     }
 
+    /// Executes the query, returning a [`watch::RowJsonCursor`] to obtain results.
+    #[cfg(feature = "watch")]
+    pub fn fetch_json<T>(mut self) -> Result<watch::RowJsonCursor<T>> {
+        self.sql.append(" FORMAT JSONEachRowWithProgress");
+
+        let response = self.do_execute(true)?;
+        Ok(watch::RowJsonCursor::new(response))
+    }
+
+    /// Executes the query and returns just a single row.
+    ///
+    /// Note that `T` must be owned.
+    #[cfg(feature = "watch")]
+    pub async fn fetch_json_one<T>(self) -> Result<T>
+    where
+        T: for<'b> Deserialize<'b>,
+    {
+        match self.fetch_json()?.next().await {
+            Ok(Some(row)) => Ok(row),
+            Ok(None) => Err(Error::RowNotFound),
+            Err(err) => Err(err),
+        }
+    }
+
+    /// Executes the query and returns at most one row.
+    ///
+    /// Note that `T` must be owned.
+    #[cfg(feature = "watch")]
+    pub async fn fetch_json_optional<T>(self) -> Result<Option<T>>
+    where
+        T: for<'b> Deserialize<'b>,
+    {
+        self.fetch_json()?.next().await
+    }
+
+    /// Executes the query and returns all the generated results,
+    /// collected into a [`Vec`].
+    ///
+    /// Note that `T` must be owned.
+    #[cfg(feature = "watch")]
+    pub async fn fetch_json_all<T>(self) -> Result<Vec<T>>
+    where
+        T: for<'b> Deserialize<'b>,
+    {
+        let mut result = Vec::new();
+        let mut cursor = self.fetch_json::<T>()?;
+
+        while let Some(row) = cursor.next().await? {
+            result.push(row);
+        }
+
+        Ok(result)
+    }
+
     /// Executes the query and returns just a single row.
     ///
     /// Note that `T` must be owned.

src/watch.rs

@@ -6,6 +6,7 @@ use sha1::{Digest, Sha1};
 use crate::{
     cursor::JsonCursor,
     error::{Error, Result},
+    response::Response,
     row::Row,
     sql::{Bind, SqlBuilder},
     Client, Compression,
@@ -165,6 +166,23 @@ impl EventCursor {
     }
 }
 
+/// A cursor that emits rows in JSON format.
+pub struct RowJsonCursor<T>(JsonCursor<T>);
+
+impl<T> RowJsonCursor<T> {
+    pub(crate) fn new(response: Response) -> Self {
+        Self(JsonCursor::new(response))
+    }
+
+    /// Emits the next row.
+    pub async fn next<'a, 'b: 'a>(&'a mut self) -> Result<Option<T>>
+    where
+        T: Deserialize<'b>,
+    {
+        self.0.next().await
+    }
+}
+
 // === RowCursor ===
 
 /// A cursor that emits `(Version, T)`.

tests/it/query.rs

@@ -263,3 +263,98 @@ async fn prints_query() {
         "SELECT ?fields FROM test WHERE a = ? AND b < ?"
     );
 }
+
+#[cfg(feature = "watch")]
+#[tokio::test]
+async fn fetches_json_row() {
+    let client = prepare_database!();
+
+    let value = client
+        .query("SELECT 1,2,3")
+        .fetch_json_one::<serde_json::Value>()
+        .await
+        .unwrap();
+
+    assert_eq!(value, serde_json::json!({ "1": 1, "2": 2, "3": 3}));
+
+    let value = client
+        .query("SELECT (1,2,3) as data")
+        .fetch_json_one::<serde_json::Value>()
+        .await
+        .unwrap();
+
+    assert_eq!(value, serde_json::json!({ "data": [1,2,3]}));
+}
+
+#[cfg(feature = "watch")]
+#[tokio::test]
+async fn fetches_json_struct() {
+    let client = prepare_database!();
+
+    #[derive(Debug, Deserialize, PartialEq)]
+    struct Row {
+        one: i8,
+        two: String,
+        three: f32,
+        four: bool,
+    }
+
+    let value = client
+        .query("SELECT -1 as one, '2' as two, 3.0 as three, false as four")
+        .fetch_json_one::<Row>()
+        .await
+        .unwrap();
+
+    assert_eq!(
+        value,
+        Row {
+            one: -1,
+            two: "2".to_owned(),
+            three: 3.0,
+            four: false,
+        }
+    );
+}
+
+#[cfg(feature = "watch")]
+#[tokio::test]
+async fn describes_table() {
+    let client = prepare_database!();
+
+    let columns = client
+        .query("DESCRIBE TABLE system.users")
+        .fetch_json_all::<serde_json::Value>()
+        .await
+        .unwrap();
+    for c in &columns {
+        println!("{c}");
+    }
+    let columns = columns
+        .into_iter()
+        .map(|row| {
+            let column_name = row
+                .as_object()
+                .expect("JSONEachRow")
+                .get("name")
+                .expect("`system.users` must contain the `name` column");
+            (column_name.as_str().unwrap().to_owned(), row)
+        })
+        .collect::<std::collections::HashMap<String, serde_json::Value>>();
+    dbg!(&columns);
+
+    let name_column = columns
+        .get("name")
+        .expect("`system.users` must contain the `name` column");
+    assert_eq!(
+        name_column.as_object().unwrap().get("type").unwrap(),
+        &serde_json::json!("String")
+    );
+
+    let id_column = columns
+        .get("id")
+        .expect("`system.users` must contain the `id` column");
+    assert_eq!(
+        id_column.as_object().unwrap().get("type").unwrap(),
+        &serde_json::json!("UUID")
+    );
+}