From 31a074c7a5635f84992e2e9839802748aad54b0f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alja=C5=BE=20Mur=20Er=C5=BEen?= <aljaz@edgedb.com>
Date: Tue, 2 Jul 2024 19:41:28 +0200
Subject: [PATCH] fix: empty relational literals

Closes #4421
---
 prqlc/prqlc/src/sql/gen_query.rs     | 28 ++++++++++-----
 prqlc/prqlc/tests/integration/sql.rs | 51 +++++++++++++++++++++++++++-
 2 files changed, 69 insertions(+), 10 deletions(-)

diff --git a/prqlc/prqlc/src/sql/gen_query.rs b/prqlc/prqlc/src/sql/gen_query.rs
index 4d13dec9fabe..2e01688b3eb5 100644
--- a/prqlc/prqlc/src/sql/gen_query.rs
+++ b/prqlc/prqlc/src/sql/gen_query.rs
@@ -425,6 +425,25 @@ fn translate_cte(cte: Cte, ctx: &mut Context) -> Result<(sql_ast::Cte, bool)> {
 fn translate_relation_literal(data: RelationLiteral, ctx: &Context) -> Result<sql_ast::Query> {
     // TODO: this could be made to use VALUES instead of SELECT UNION ALL SELECT
     //       I'm not sure about compatibility though.
+    // edit: probably not, because VALUES has no way of setting names of the columns
+    //       Postgres will just name them column1, column2 as so on.
+    //       Which means we can use VALUES, but only if this is not the top-level statement,
+    //       where they really matter.
+
+    if data.rows.is_empty() {
+        return Ok(default_query(sql_ast::SetExpr::Select(Box::new(Select {
+            projection: data
+                .columns
+                .iter()
+                .map(|col_name| SelectItem::ExprWithAlias {
+                    expr: sql_ast::Expr::Value(sql_ast::Value::Null),
+                    alias: translate_ident_part(col_name.clone(), ctx),
+                })
+                .collect(),
+            selection: Some(sql_ast::Expr::Value(sql_ast::Value::Boolean(false))),
+            ..default_select()
+        }))));
+    }
 
     let mut selects = Vec::with_capacity(data.rows.len());
 
@@ -444,15 +463,6 @@ fn translate_relation_literal(data: RelationLiteral, ctx: &Context) -> Result<sq
         selects.push(body)
     }
 
-    if selects.is_empty() {
-        return Err(
-            Error::new_simple("No rows provided for `from_text`".to_string()).push_hint(
-                "add a newline, then a row of data following the column. If using \
-                the json format, ensure `data` isn't empty",
-            ),
-        );
-    }
-
     let mut body = selects.remove(0);
     for select in selects {
         body = SetExpr::SetOperation {
diff --git a/prqlc/prqlc/tests/integration/sql.rs b/prqlc/prqlc/tests/integration/sql.rs
index 9fcb2cd03b66..34086fa78c3b 100644
--- a/prqlc/prqlc/tests/integration/sql.rs
+++ b/prqlc/prqlc/tests/integration/sql.rs
@@ -4194,7 +4194,7 @@ fn test_name_inference() {
 }
 
 #[test]
-fn test_from_text() {
+fn test_from_text_01() {
     assert_snapshot!(compile(r#"
     from_text format:csv """
 a,b,c
@@ -4223,7 +4223,10 @@ a,b,c
       table_0
     "###
     );
+}
 
+#[test]
+fn test_from_text_02() {
     assert_snapshot!(compile(r#"
     from_text format:json '''
       [{"a": 1, "b": "x", "c": false }, {"a": 4, "b": "y", "c": null }]
@@ -4250,7 +4253,10 @@ a,b,c
       table_0
     "###
     );
+}
 
+#[test]
+fn test_from_text_03() {
     assert_snapshot!(compile(r#"
     std.from_text format:json '''{
         "columns": ["a", "b", "c"],
@@ -4283,6 +4289,49 @@ a,b,c
     );
 }
 
+#[test]
+fn test_from_text_04() {
+    assert_snapshot!(compile(r#"
+    std.from_text 'a,b'
+    "#).unwrap(),
+        @r###"
+    WITH table_0 AS (
+      SELECT
+        NULL AS a,
+        NULL AS b
+      WHERE
+        false
+    )
+    SELECT
+      a,
+      b
+    FROM
+      table_0
+    "###
+    );
+
+    assert_snapshot!(compile(r#"
+    std.from_text format:json '''{"columns": ["a", "b", "c"], "data": []}'''
+    "#).unwrap(),
+        @r###"
+    WITH table_0 AS (
+      SELECT
+        NULL AS a,
+        NULL AS b,
+        NULL AS c
+      WHERE
+        false
+    )
+    SELECT
+      a,
+      b,
+      c
+    FROM
+      table_0
+    "###
+    );
+}
+
 #[test]
 fn test_header() {
     // Test both target & version at the same time