PERF-#7230: Don't preserve bad partition for merge

[local-dev-korbit-ai-mentor]

What do these changes do?

merge preserves the row splitting, but sometimes it's better to repartition. One extremely bad case is when there is a sequence of heavyweight operations where inefficient partitioning persists from the first to the last. For example, sequence of merge operations, where the left operand of the very first operation in the chain has only one partition. For example:

import modin.pandas as pd
import numpy as np
from time import time
from modin.utils import execute
import modin.config as cfg

cfg.NPartitions.put(8)


pd.DataFrame(np.arange(cfg.NPartitions.get() * cfg.MinPartitionSize.get())).to_numpy()

for _ in range(3):
    np.random.seed(42)
    # small
    df1 = pd.DataFrame(np.random.randint(100, size=(32, 16)))
    assert df1._query_compiler._modin_frame._partitions.shape == (1, 1)

    # big
    df2 = pd.DataFrame(np.random.randint(100, size=(16_000, 16)))
    df3 = pd.DataFrame(np.random.randint(100, size=(200_000, 16)))

    start = time()

    res = df1.merge(df2, on=2)
    res = res.merge(df3, left_on='3_x', right_on=3)
    print(res.shape)
    execute(res)

    print(f"time: {time()-start}")

Results: 5.07 sec (on main) vs 2.56 sec (in the PR)

• first commit message and PR title follow format outlined here

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• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
• signed commit with git commit -s
• Resolves #7230
• tests added and passing
• module layout described at docs/development/architecture.rst is up-to-date

Description by Korbit AI

What change is being made?

Add logic to avoid preserving bad partitions during the merge operation and introduce a new test to validate this behavior.

Why are these changes being made?

The change ensures that partitions are rebalanced when the ratio of existing partitions to the ideal number of partitions falls below a threshold, improving performance and avoiding empty partitions. The new test verifies that the merge operation correctly triggers repartitioning when necessary.

[local-dev-korbit-ai-mentor]

Clone of the PR modin-project/modin#7229

[korbit-ai]

My review is in progress 📖 - I will have feedback for you in a few minutes!

[coderabbitai]

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[korbit-ai]

I have reviewed your code and did not find any issues!

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[furwellness]

/review

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PR Reviewer Guide 🔍

⏱️ Estimated effort to review: 3 🔵🔵🔵⚪⚪

🧪 PR contains tests

🔒 No security concerns identified

⚡ Key issues to review Performance Optimization The PR introduces a repartitioning step in the merge operation when the number of existing partitions is significantly lower than the ideal number. This change aims to improve performance but may need further testing to ensure it doesn't introduce unexpected behavior. Test Coverage A new test case has been added to verify the behavior of merge operation with bad partitioning. While this is a positive addition, it might be beneficial to include more comprehensive tests covering different scenarios and edge cases.