2010YOUY01 opened a new pull request, #17837:
URL: https://github.com/apache/datafusion/pull/17837
## Which issue does this PR close?
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part of #17789
## Rationale for this change
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`string_agg` is slow, see the tracking issue for details.
This PR added a new `Accumulator` implementation for the simple case: if
there is no `DISTINCT` and `ORDER BY` like `string_agg(distinct str, ',' ORDER
BY str)`, use `SimpleStringAggAccumulator` instead. The original
`StringAggAccumulator` is used for the general case with potential `DISTINCT`
and `ORDER BY`s.
While @vegarsti is working on a `GroupsAccumulator` solution that can
further speed it up potentially, I think this PR is still necessary because the
single group case like `select string_agg(str, ',') from t1` is still using the
`Accumulator` interface instead of `GroupsAccumulator`
I haven't checked the original implementation yet, and I don't know why is
it so slow. It's using `array_agg` internally, and memory bloat can be
observed. I guess the reason is redundant transcoding, and incorrect operations
on `StringView` buffers.
There are several ongoing work to improve `arrray_agg`:
https://github.com/apache/datafusion/issues/17829
### Benchmark
It's around 1000x faster. See the original issue for data setup, scaling
factor 0.1 is used for the table.
```
DataFusion CLI v50.0.0
> CREATE EXTERNAL TABLE partsupp
STORED AS PARQUET
LOCATION '/Users/yongting/Code/datafusion-sqlstorm/data/partsupp.parquet';
> select ps_partkey, string_agg(ps_comment, ';')
from partsupp
group by ps_partkey;
```
Before: ~50s
PR: 0.05s
## What changes are included in this PR?
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sometimes worth providing a summary of the individual changes in this PR.
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1. Implemented a `SimpleStringAggAccumulator` for `string_agg`
2. In the aggregate function implementation, opt for the new accumulator if
there is no `DISTINCT` and `ORDER BY` in the `string_agg()` aggregate function.
## Are these changes tested?
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Existing tests
## Are there any user-facing changes?
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