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Document behavior of INSERT with RETURNING through tests #597

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Oct 2, 2025
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Document behavior of INSERT with RETURNING through tests #597

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71 changes: 71 additions & 0 deletions crates/duckdb/src/statement.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -820,6 +820,77 @@ mod test {
Ok(())
}

// When using RETURNING clauses, DuckDB core treats the statement as a query result instead of a modification
// statement. This causes execute() to return 0 changed rows and insert() to fail with an error.
// This test demonstrates current behavior and proper usage patterns for RETURNING clauses.
#[test]
fn test_insert_with_returning_clause() -> Result<()> {
let db = Connection::open_in_memory()?;
db.execute_batch(
"CREATE SEQUENCE location_id_seq START WITH 1 INCREMENT BY 1;
CREATE TABLE location (
id INTEGER PRIMARY KEY DEFAULT nextval('location_id_seq'),
name TEXT NOT NULL
)",
)?;

// INSERT without RETURNING using execute
let changes = db.execute("INSERT INTO location (name) VALUES (?)", ["test1"])?;
assert_eq!(changes, 1);

// INSERT with RETURNING using execute - returns 0 (known limitation)
let changes = db.execute("INSERT INTO location (name) VALUES (?) RETURNING id", ["test2"])?;
assert_eq!(changes, 0);

// Verify the row was actually inserted despite returning 0
let count: i64 = db.query_row("SELECT COUNT(*) FROM location", [], |r| r.get(0))?;
assert_eq!(count, 2);

// INSERT without RETURNING using insert
let mut stmt = db.prepare("INSERT INTO location (name) VALUES (?)")?;
stmt.insert(["test3"])?;

// INSERT with RETURNING using insert - fails (known limitation)
let mut stmt = db.prepare("INSERT INTO location (name) VALUES (?) RETURNING id")?;
let result = stmt.insert(["test4"]);
assert!(matches!(result, Err(Error::StatementChangedRows(0))));

// Verify the row was still inserted despite the error
let count: i64 = db.query_row("SELECT COUNT(*) FROM location", [], |r| r.get(0))?;
assert_eq!(count, 4);

// Proper way to use RETURNING - with query_row
let id: i64 = db.query_row("INSERT INTO location (name) VALUES (?) RETURNING id", ["test5"], |r| {
r.get(0)
})?;
assert_eq!(id, 5);

// Proper way to use RETURNING - with query_map
let mut stmt = db.prepare("INSERT INTO location (name) VALUES (?) RETURNING id")?;
let ids: Vec<i64> = stmt
.query_map(["test6"], |row| row.get(0))?
.collect::<Result<Vec<_>>>()?;
assert_eq!(ids.len(), 1);
assert_eq!(ids[0], 6);

// Proper way to use RETURNING - with query_one
let id: i64 = db
.prepare("INSERT INTO location (name) VALUES (?) RETURNING id")?
.query_one(["test7"], |r| r.get(0))?;
assert_eq!(id, 7);

// Multiple RETURNING columns
let (id, name): (i64, String) = db.query_row(
"INSERT INTO location (name) VALUES (?) RETURNING id, name",
["test8"],
|r| Ok((r.get(0)?, r.get(1)?)),
)?;
assert_eq!(id, 8);
assert_eq!(name, "test8");

Ok(())
}

#[test]
fn test_exists() -> Result<()> {
let db = Connection::open_in_memory()?;
Expand Down