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ARROW-12214: [Rust][DataFusion] Add tests for limit #9897

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ARROW-12214: [Rust][DataFusion] Add tests for limit #9897

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ARROW-12214: [Rust][DataFusion] Add tests for limit
alamb Apr 5, 2021
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fix fmt
alamb Apr 5, 2021
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40 changes: 40 additions & 0 deletions rust/datafusion/src/execution/context.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1721,6 +1721,46 @@ mod tests {
Ok(())
}

#[tokio::test]
async fn limit() -> Result<()> {
let tmp_dir = TempDir::new()?;
let mut ctx = create_ctx(&tmp_dir, 1)?;
ctx.register_table("t", table_with_sequence(1, 1000).unwrap())
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@Dandandan Dandandan Apr 6, 2021

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I was looking at Limit yesterday - it seems to poll new batches even when the limit has been reached (and throw away the result in the end)? Not wrong - but quite inefficient of course :)

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@alamb alamb Apr 6, 2021

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I am not sure --- I have a program (https://github.com/influxdata/influxdb_iox/pull/1117) that I made for testing some stuff as we roll out IOx and there is definitely a problem with limit, though I haven't figured out if it is a problem in DataFusion or not

For this plan:

> explain select * from chunks limit 1;
+--------------+-----------------------------------------------------------------------------------------------------------------------------------------+
| plan_type    | plan                                                                                                                                    |
+--------------+-----------------------------------------------------------------------------------------------------------------------------------------+
| logical_plan | Limit: 1                                                                                                                                |
|              |   Projection: #database_name, #id, #partition_key, #storage, #estimated_bytes, #time_of_first_write, #time_of_last_write, #time_closing |
|              |     TableScan: chunks projection=None                                                                                                   |
+--------------+-----------------------------------------------------------------------------------------------------------------------------------------+
1 rows in set. Query took 0 seconds.

Produces no rows (and no columns) when there is a limit:

> select * from chunks limit 1;
++
||
++
++
0 rows in set. Query took 0 seconds.

But there is definitely data in that table:

> select * from chunks;
+-----------------------------------+-----+---------------------+---------------------+-----------------+-------------------------------+-------------------------------+-------------------------------+
| database_name                     | id  | partition_key       | storage             | estimated_bytes | time_of_first_write           | time_of_last_write            | time_closing                  |
+-----------------------------------+-----+---------------------+---------------------+-----------------+-------------------------------+-------------------------------+-------------------------------+
| 844910ece80be8bc_05a7a51565539000 | 0   | 2021-04-05 21:00:00 | OpenMutableBuffer   | 259733          | 2021-04-05 21:29:38.978576237 | 2021-04-05 21:49:47.995408514 |                               |
....
| 844910ece80be8bc_eaec8df57a81a1e9 | 1   | 2021-04-05 21:00:00 | OpenMutableBuffer   | 6408933         | 2021-04-05 21:44:31.507950286 | 2021-04-05 21:55:37.226659960 |                               |
+-----------------------------------+-----+---------------------+---------------------+-----------------+-------------------------------+-------------------------------+-------------------------------+
226 rows in set. Query took 0 seconds.

I am still looking into it

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@alamb alamb Apr 7, 2021

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https://issues.apache.org/jira/browse/ARROW-12235 is one possibly problem I found

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@alamb alamb Apr 7, 2021

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I was looking at Limit yesterday - it seems to poll new batches even when the limit has been reached (and throw away the result in the end)? Not wrong - but quite inefficient of course :)

@Dandandan I spent some time looking into this and you were absolutely correct. I have a PR (draft) up with a proposed fix: #9926

.unwrap();

let results =
plan_and_collect(&mut ctx, "SELECT i FROM t ORDER BY i DESC limit 3")
.await
.unwrap();

let expected = vec![
"+------+", "| i |", "+------+", "| 1000 |", "| 999 |", "| 998 |",
"+------+",
];

assert_batches_eq!(expected, &results);

let results = plan_and_collect(&mut ctx, "SELECT i FROM t ORDER BY i limit 3")
.await
.unwrap();

let expected = vec![
"+---+", "| i |", "+---+", "| 1 |", "| 2 |", "| 3 |", "+---+",
];

assert_batches_eq!(expected, &results);

let results = plan_and_collect(&mut ctx, "SELECT i FROM t limit 3")
.await
.unwrap();

// the actual rows are not guaranteed, so only check the count (should be 3)
let num_rows: usize = results.into_iter().map(|b| b.num_rows()).sum();
assert_eq!(num_rows, 3);

Ok(())
}

#[tokio::test]
async fn case_sensitive_identifiers_functions() {
let mut ctx = ExecutionContext::new();
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