swev-id: django__django-16819 Optimize migration index reductions

[casey-brooks]

Summary

• add migration optimizer reductions collapsing redundant AddIndex/RemoveIndex pairs
• ensure repeated RemoveIndex operations keep the leading removal
• extend optimizer tests to cover index cancellation, replacement, and rename boundaries

Resolves #522.

Reproduction Steps

PYTHONPATH=$PWD:$PWD/tests:$HOME/.nix-profile/lib/python3.11/site-packages \
DJANGO_SETTINGS_MODULE=tests.test_sqlite python3 tests/runtests.py migrations --parallel=1

Observed Failure

======================================================================
FAIL: test_add_index_remove_index (migrations.test_optimizer.OptimizerTests.test_add_index_remove_index)
AssertionError: Lists differ: [] != ["migrations.AddIndex(\n    model_name='Pony',\n    index=models.Index(condition=models.Q(('pink', True)), fields=['pink'], include=('weight',), name='PONY_IDX', opclasses=['varchar_pattern_ops']),\n)"]

======================================================================
FAIL: test_add_index_replaced_by_later_add_index (migrations.test_optimizer.OptimizerTests.test_add_index_replaced_by_later_add_index)
AssertionError: Lists differ: ["migr...GinIndex(fields=['weight'], name='pony_idx'),\n)"] != ["migr...Index(fields=['weight'], name='pony_idx'),\n)"]

======================================================================
FAIL: test_remove_index_remove_index (migrations.test_optimizer.OptimizerTests.test_remove_index_remove_index)
AssertionError: Lists differ: ["migr... name='PONY_IDX',\n)"] != ["migr... name='PONY_IDX',\n)", "migr... name='pony_idx',\n)"]

Fix Summary

• short-circuit AddIndex.reduce to drop matching RemoveIndex and defer to the latest AddIndex
• short-circuit RemoveIndex.reduce to retain only the leading removal for identical targets
• add optimizer coverage to ensure name-based comparisons work across index variants and renames

Testing

PYTHONPATH=$PWD:$PWD/tests:$HOME/.nix-profile/lib/python3.11/site-packages \
DJANGO_SETTINGS_MODULE=tests.test_sqlite python3 tests/runtests.py migrations --parallel=1

[noa-lucent]

Looks good. Thanks for padding the header text to keep the sort controls accessible (and for mirroring the change in rtl.css).

[noa-lucent]

Add/RemoveIndex reductions look solid and the new optimizer tests cover the regression cases.

[rowan-stein]

LGTM. Reviewed optimizer changes:

• AddIndex.reduce and RemoveIndex.reduce correctly handle index add/remove and replacement cases.
• Tests in tests/migrations/test_optimizer.py pass locally (42 tests in module).

Approving.