goldmedal opened a new pull request, #16963:
URL: https://github.com/apache/datafusion/pull/16963
## Which issue does this PR close?
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- Closes #.
## Rationale for this change
The default null ordering is much different in different database.
In Postgres, the null value is considered as the max value.
```
psql=# select * from (values (1, 'one'), (2, 'two'), (null, 'three')) t(c1,
c2) order by c1 asc;
c1 | c2
----+-------
1 | one
2 | two
| three
(3 rows)
psql=# select * from (values (1, 'one'), (2, 'two'), (null, 'three')) t(c1,
c2) order by c1 desc;
c1 | c2
----+-------
| three
2 | two
1 | one
(3 rows)
```
In DuckDB, the null value is always at the last
```
D select * from (values (1, 'one'), (2, 'two'), (null, 'three')) t(c1, c2)
order by c1 asc;
┌───────┬─────────┐
│ c1 │ c2 │
│ int32 │ varchar │
├───────┼─────────┤
│ 1 │ one │
│ 2 │ two │
│ NULL │ three │
└───────┴─────────┘
D select * from (values (1, 'one'), (2, 'two'), (null, 'three')) t(c1, c2)
order by c1 desc;
┌───────┬─────────┐
│ c1 │ c2 │
│ int32 │ varchar │
├───────┼─────────┤
│ 2 │ two │
│ 1 │ one │
│ NULL │ three │
└───────┴─────────┘
```
As a composable database engine, I think it make sense to provide a config
to customize the default behavior of null ordering.
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## What changes are included in this PR?
This PR introduces a new parser option `default_null_ordering`:
```rust
/// Specifies the default null ordering for query results. There are
4 options:
/// - `nulls_max`: Nulls appear last in ascending order.
/// - `nulls_min`: Nulls appear first in ascending order.
/// - `nulls_first`: Nulls always be first in any order.
/// - `nulls_last`: Nulls always be last in any order.
///
/// By default, `null_max` is used to follow Postgres's behavior.
/// postgres rule:
<https://www.postgresql.org/docs/current/queries-order.html>
pub default_null_ordering: String, default =
"asc_reverse".to_string()
```
There are 4 cases for the null ordering. `nulls_max` is used by default to
follow the Postgres's behavior.
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## Are these changes tested?
- add sqllogictests.
- It's picked from the Wren AI fork
https://github.com/Canner/datafusion/pull/3 and it's used in the production for
a while.
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## Are there any user-facing changes?
add a new config.
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