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ARROW-12214: [Rust][DataFusion] Add tests for limit - #9897
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| 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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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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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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https://issues.apache.org/jira/browse/ARROW-12235 is one possibly problem I found
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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
I was debugging another issue (not a bug in DataFusion I don't think) but noticed there wasn't any coverage for LIMIT in exec.rs, so I figured I would add some.
(well really I was writing a test to trigger what I thought was a bug in DataFusion -- lol)