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[fix] select * on empty table #613

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Merged
merged 2 commits into from
Jun 23, 2021
Merged

[fix] select * on empty table #613

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4ab0d40
[fix] select * on empty table
rdettai Jun 23, 2021
0023c05
clippy
rdettai Jun 23, 2021
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15 changes: 14 additions & 1 deletion datafusion/src/execution/context.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -893,7 +893,7 @@ mod tests {
logical_plan::{col, create_udf, sum, Expr},
};
use crate::{
datasource::{MemTable, TableType},
datasource::{empty::EmptyTable, MemTable, TableType},
logical_plan::create_udaf,
physical_plan::expressions::AvgAccumulator,
};
Expand Down Expand Up @@ -3333,6 +3333,19 @@ mod tests {
assert_batches_sorted_eq!(expected, &result);
}

#[tokio::test]
async fn query_empty_table() {
let mut ctx = ExecutionContext::new();
let empty_table = Arc::new(EmptyTable::new(Arc::new(Schema::empty())));
ctx.register_table("test_tbl", empty_table).unwrap();
let sql = "SELECT * FROM test_tbl";
let result = plan_and_collect(&mut ctx, sql)
.await
.expect("Query empty table");
let expected = vec!["++", "++"];
assert_batches_sorted_eq!(expected, &result);
}

struct MyPhysicalPlanner {}

impl PhysicalPlanner for MyPhysicalPlanner {
Expand Down
5 changes: 3 additions & 2 deletions datafusion/src/optimizer/projection_push_down.rs
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Expand Up @@ -83,9 +83,10 @@ fn get_projected_schema(
.collect();

if projection.is_empty() {
if has_projection {
if has_projection && !schema.fields().is_empty() {
// Ensure that we are reading at least one column from the table in case the query
// does not reference any columns directly such as "SELECT COUNT(1) FROM table"
// does not reference any columns directly such as "SELECT COUNT(1) FROM table",
// except when the table is empty (no column)
projection.push(0);
} else {
// for table scan without projection, we default to return all columns
Expand Down