Overview

Native write paths in Spark 3.2/3.3 and the ClickHouse MergeTree writer
recompute the __bucket_value__ expression even when a precomputed
attribute already exists. This adds unnecessary overhead and complicates
downstream projections.

Steps to Reproduce

• Write bucketed data using the native writer.
• Inspect the execution plan; the bucket ID is projected multiple times
  instead of reusing a single attribute.

Expected Behavior

The bucket ID should be computed once (e.g., in an initial ProjectExec)
and reused by subsequent stages.

Actual Behavior

Every stage re-evaluates the bucket expression, leading to redundant
projections and performance overhead.

Impact

• Increased CPU time for bucketed writes.
• Harder-to-read execution plans with repetitive projections.

Proposed Fix

• Guard projections in Spark 3.2/3.3 shims and ClickHouse MergeTree writer
  so they append __bucket_value__ only when missing.
• Store the computed bucket ID in an attribute and reuse it downstream.