fix: ORDER BY/window partition preserves prior boundaries on NULL & const keys

[aunjgr]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #24248

What this PR does / why we need it:

partition.Partition is invoked once per sort key from
pkg/sql/colexec/order/order.go::sortAndSend (and per partition spec
from pkg/sql/colexec/window/window.go), reusing the same diffs
slice across calls. The contract is that each call must OR new
boundaries onto diffs — never overwrite an existing true.

Two code paths in pkg/partition/partition.go violated that contract:

1. Both-NULL overwrite. In genericPartition / bytesPartition,
   when both adjacent rows were NULL under the current key the code
   did diffs[i] = false, erasing a boundary set by a prior key.
   This made multi-key ORDER BY over a FULL OUTER JOIN produce rows
   out of order: with the primary key NULL on the right-padded side,
   the secondary key was never sub-sorted within the t1.s = NULL
   partition. Repro from the issue:

   create table t1(s int, v varchar(5));
   create table t2(s int, v varchar(5));
   insert into t1 values (1,'a'),(5,'b'),(NULL,'x');
   insert into t2 values (13,'c'),(14,NULL);
   select t1.s, t1.v, t2.s, t2.v
     from t1 full outer join t2 on t1.s = t2.s
     order by t1.s, t2.s, t1.v, t2.v;

   Before this PR, (NULL,NULL,14,NULL) was emitted before
   (NULL,NULL,13,'c'). After: 13 before 14, as expected.

2. Const-vector boundary collapse. When vec.IsConst() the code
   returned partitions = [0] and, for IsConstNull(), even cleared
   all of diffs. A const key appearing mid-list therefore collapsed
   all previously found partitions in the same Partition call series.

Fix

In both genericPartition and bytesPartition, treat every "rows are
equal under this key" outcome as a no-op on diffs (the both-NULL
case and the entire const branch). Always rebuild partitions from
the OR-accumulated diffs at the end. diffs[0] = true is set up
front, so the const-vector path now naturally yields [0] via the
final scan rather than via a special branch.

This is the same semantics the non-const non-null path already had
(diffs[i] = diffs[i] || (v != w)).

Tests

• pkg/partition/partition_test.go::TestPartitionAccumulatesDiffs
  exercises the reuse contract — multiple successive Partition
  calls on the same diffs/partitions slices — across all four
  problematic shapes:
  • fixed-width second key with all NULLs preserves the first key's
    boundaries
  • fixed-width const non-null second key preserves boundaries
  • fixed-width const NULL second key preserves boundaries
  • bytes second key with all NULLs preserves boundaries
  • bytes const second key preserves boundaries
  • sanity check that two non-trivial keys union their boundaries
• test/distributed/cases/join/fullouterjoin.sql gets the issue's
  exact repro and a NULLS LAST (DESC) secondary-key variant.
• mo-tester -m run on the test file: 45/45 SUCCESS, 0 FAILED.
• Unit tests for pkg/sql/colexec/order/...,
  pkg/sql/colexec/window/..., and pkg/partition/... all pass.

PR Type and Checklists

Standard Checklist:

• Pull request title is following Conventional Commits syntax.
• Tests added/updated.
• Documentation is not required (internal correctness fix; user-visible behavior now matches a standard-compliant engine).

Checklist for BUG PR Type:

• I have added test cases to cover the new code.

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[Copilot]

Pull request overview

Fixes multi-key ORDER BY / window partition boundary handling when partition.Partition is called repeatedly with a reused diffs slice, ensuring previously-discovered boundaries are preserved for NULL-vs-NULL and const-vector keys (regression #24248).

Changes:

• Update genericPartition and bytesPartition to never overwrite existing diffs[i]==true with false for “both NULL” and const-vector paths, and always rebuild partitions from accumulated diffs.
• Add unit coverage to verify the “accumulate diffs across successive Partition calls” contract.
• Add a distributed regression case for FULL OUTER JOIN + multi-key ORDER BY with NULLs.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
pkg/partition/partition.go	Preserves prior boundaries in diffs for NULL/const keys and rebuilds partitions from the accumulated diffs.
pkg/partition/partition_test.go	Adds regression/unit tests covering successive Partition calls that reuse the same diffs/partitions slices.
test/distributed/cases/join/fullouterjoin.sql	Adds an end-to-end FULL OUTER JOIN + ORDER BY repro for #24248 (including a DESC variant).
test/distributed/cases/join/fullouterjoin.result	Updates expected output for the new distributed regression queries.