Allow to set up the default null ordering

[goldmedal]

Which issue does this PR close?

• Closes #.

Rationale for this change

To fit different requirements of default null ordering, this PR introduces a new config to set up the default null ordering when parsing SQL.

        /// Specifies the default null ordering for query results
        /// By default, `asc_reverse` 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:

/// Represents the null ordering for sorting expressions.
#[derive(Debug, Clone, Copy)]
pub enum NullOrdering {
    /// Ascending order with nulls appearing last.
    AscReverse,
    /// Descending order with nulls appearing last.
    DescReverse,
    /// Nulls appear first.
    NullsFirst,
    /// Nulls appear last.
    NullsLast,
}

Postgres is AscReverse. So we set it by default to follow the PostgreSQL behavior.
DuckDB is NullsLast. If users want to follow it, they can configure it to NullsLast easily.

What changes are included in this PR?

add new config.

Are these changes tested?

add sqllogictest

Are there any user-facing changes?

[Copilot]

Pull Request Overview

This PR introduces a configurable default null ordering for SQL queries to accommodate different database behaviors. Instead of hardcoded PostgreSQL-style null ordering, users can now specify their preferred default null ordering strategy.

• Adds a new default_null_ordering configuration option with four possible values: asc_reverse, desc_reverse, nulls_first, and nulls_last
• Updates the SQL parser to use the configured null ordering when explicit NULLS FIRST/LAST clauses are not specified
• Maintains PostgreSQL compatibility by defaulting to asc_reverse behavior

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
datafusion/common/src/config.rs	Adds the new default_null_ordering configuration option
datafusion/sql/src/planner.rs	Defines the NullOrdering enum and implements string parsing logic
datafusion/sql/src/expr/order_by.rs	Updates ORDER BY expression handling to use configurable null ordering
datafusion/sql/src/statement.rs	Updates window function ordering to use configurable null ordering
datafusion/core/src/execution/session_state.rs	Integrates the configuration into session state
datafusion/sql/tests/sql_integration.rs	Updates test cases to include the new configuration parameter
datafusion/sqllogictest/test_files/order.slt	Adds comprehensive test coverage for all null ordering configurations
datafusion/sqllogictest/test_files/information_schema.slt	Updates information schema output to include the new configuration

[goldmedal]

Let's ignore the MSRV check. DataFusion 46.0.0 is too old. We should move to the latest version as soon as possible.

[douenergy]

The NULLS FIRST and NULLS LAST options can be used to determine whether nulls appear before or after non-null values in the sort ordering. By default, null values sort as if larger than any non-null value; that is, NULLS FIRST is the default for DESC order, and NULLS LAST otherwise. -- Postgres doc

TLDR

desc_reverse flips the default NULL placement: ASC gives NULLS FIRST, DESC gives NULLS LAST; opposite of standard.
asc_reverse flips the default NULL placement: ASC gives NULLS LAST, DESC gives NULLS FIRST; matches the standard SQL behavior.

[douenergy]

Thanks @goldmedal