`EnforceDistribution` replaces `SortPreservingMergeExec` with `CoalescePartitionsExec` when parent maintains input order

[rkrishn7]

Is your feature request related to a problem or challenge?

When EnforceDistribution processes a child that requires SinglePartition or HashPartitioned distribution but has no ordering requirement, it unconditionally calls replace_order_preserving_variants, replacing SortPreservingMergeExec with CoalescePartitionsExec. This destroys ordering even when the parent declares maintains_input_order = true, meaning the ordering would have flowed through to ancestors that may benefit from it.

For example, consider a plan where a custom operator (like a remote execution node) requires SinglePartition input and maintains input order, nested inside UnionExec branches:

SortRequiredExec [col ASC]
    UnionExec
        RemoteExec                          # requires SinglePartition, maintains order
            DataSourceExec (N partitions, sorted [col ASC])
        RemoteExec
            DataSourceExec (N partitions, sorted [col ASC])

EnforceDistribution correctly inserts SortPreservingMergeExec to satisfy the SinglePartition requirement while preserving per-partition sort ordering. But it then immediately replaces SPM with CoalescePartitionsExec, destroying the ordering. This cascades upward — UnionExec now has unordered children, and EnforceSorting must insert a full SortExec to re-establish ordering, which is significantly more expensive than the merge that was removed.

The UnspecifiedDistribution branch already correctly checks maintains_input_order before replacing order-preserving variants, but SinglePartition || HashPartitioned does not.

Describe the solution you'd like

Add a !maintains guard to the SinglePartition || HashPartitioned branch in ensure_distribution, aligning it with the existing UnspecifiedDistribution branch:

Distribution::SinglePartition | Distribution::HashPartitioned(_) => {
      if !maintains {
          child = replace_order_preserving_variants(child)?;
      }
}

This ensures that when a parent maintains input order, SortPreservingMergeExec is preserved so ordering can propagate to ancestors that need it.

Describe alternatives you've considered

No response

Additional context

This issue is particularly impactful when prefer_existing_sort is enabled. With prefer_existing_sort = true, the optimizer avoids stripping order-preserving variants at ancestor nodes that have ordering requirements. This means the ordering preserved by SPM inside each branch would successfully flow through the entire tree — but only if SPM survives the EnforceDistribution pass.

There is more context from the specific case that prompted this here: massive-com#35

[rkrishn7]

take