CometPlainVector: cache offsetBufferAddress for variable-width vectors

[mbutrovich]

What is the problem the feature request solves?

CometPlainVector.getUTF8String(int) and CometPlainVector.getBinary(int) dereference the offset buffer address on every call:

long offsetBufferAddress = varWidthVector.getOffsetBuffer().memoryAddress();
int offset = Platform.getInt(null, offsetBufferAddress + rowId * 4L);

That is one Arrow method call, one buffer dereference, and one memoryAddress call per row, duplicating work valueBufferAddress already avoids (the value buffer address is cached as a final long field in the constructor).

For workloads that read VarChar or VarBinary columns heavily through CometPlainVector (native scan, shuffle, and the JVM UDF dispatch path introduced in #4267), the per-row overhead is pure bookkeeping.

Describe the potential solution

Hoist the offset buffer address into a final long offsetBufferAddress field set in the constructor alongside valueBufferAddress, and use it directly in getUTF8String / getBinary:

private final long offsetBufferAddress;

public CometPlainVector(ValueVector vector, boolean useDecimal128, boolean isUuid, boolean isReused) {
  // ...
  if (vector instanceof BaseVariableWidthVector) {
    this.offsetBufferAddress = ((BaseVariableWidthVector) vector).getOffsetBuffer().memoryAddress();
  } else {
    this.offsetBufferAddress = -1;
  }
}

public UTF8String getUTF8String(int rowId) {
  if (isNullAt(rowId)) return null;
  if (!isBaseFixedWidthVector) {
    int offset = Platform.getInt(null, offsetBufferAddress + rowId * 4L);
    int length = Platform.getInt(null, offsetBufferAddress + (rowId + 1L) * 4L) - offset;
    return UTF8String.fromAddress(null, valueBufferAddress + offset, length);
  }
  // fixed-width branch unchanged
}

Same pattern for getBinary. JIT treats the address as a register-cached constant across the hot loop.

Additional context

• valueBufferAddress already uses this pattern; this proposal extends it for symmetry and for the same reason it was used there.
• Arrow guarantees getOffsetBuffer() returns the same buffer object for the lifetime of the vector, so caching the address at construction is safe.
• No API surface change. No behavior change. Strictly reduces per-row work for variable-width reads.
• Benefits every current caller of CometPlainVector, not just one code path.

[0lai0]

take