API, Core: Add toString to StructProjection and StructLikeUtil

[anoopj]

StructProjection and the struct returned by StructLikeUtil.copy represent partition tuples but relied on the default Object.toString, which prints an unreadable identity hash when debugging. Render field names and values instead.

Followup from #18108

[amogh-jahagirdar]

Thanks for this, I agree we need a more human readable name for these structs

[dramaticlly]

Looks like we already used StringJoiner before in PartitionSet

iceberg/core/src/main/java/org/apache/iceberg/util/PartitionSet.java

Line 194 in 574e446

StringJoiner result = new StringJoiner(", ", "[", "]");

, so it shall save some manual stringBuilder logic. Might worth considering something like below

Suggested change

	StringBuilder sb = new StringBuilder();
	sb.append("StructProjection{");
	List<Types.NestedField> fields = type.fields();
	for (int i = 0; i < fields.size(); i += 1) {
	if (i > 0) {
	sb.append(", ");
	}
	sb.append(fields.get(i).name()).append("=").append(get(i, Object.class));
	}
	sb.append("}");
	return sb.toString();
	List<Types.NestedField> fields = type.fields();
	StringJoiner joiner = new StringJoiner(", ", "StructProjection{", "}");
	for (int pos = 0; pos < fields.size(); pos += 1) {
	joiner.add(fields.get(pos).name() + "=" + get(pos, Object.class));
	}
	return joiner.toString();

On a separate note, I noticed that if we use get(pos, Object.class) on deeply nested struct, it might have side effect of invoking javaClass.cast(nestedProjections[pos].wrap(nestedStruct)), so worth double checking whether we want the behavior as part of toString call.

[amogh-jahagirdar]

it might have side effect of invoking javaClass.cast(nestedProjections[pos].wrap(nestedStruct)),

This is a great callout, I missed this originally but any reason we can't just read from the actual struct? Otherwise logging one of these may lead to really unexpected results after the logging as a result of the side effect.

[anoopj]

Thanks for the feedback. I changed it such that we are reading it from the original struct.

[dramaticlly]

I am wondering if helps to add a few tests in https://github.com/apache/iceberg/blob/main/api/src/test/java/org/apache/iceberg/util/TestStructProjection.java

Given the current form, the structProjection toString only show the top level field, which might not help much for the human-readable debug-ability. I do feel that the toString is effectively a nested materialization for struct projection, so probing is not free.

StructProjection.create(PROJECTED_STRUCT, PROJECTED_STRUCT).wrap(Row.of(1L, Row.of("John", "Q", "Doe"))).toString()  
//return  "StructProjection{id=1, person=org.apache.iceberg.TestHelpers$Row@447e510}"
//  instead of desired
// "StructProjection{id=1, person=StructProjection{first=John, middle=Q, last=Doe}}"

[anoopj]

I think both are good points. I initially hesitated to add tests because currently we don't have many toString tests in the project.

I made toString descend into nested projections. To avoid the wrap() side effect we discussed above, the nested descent uses copyFor, which wraps a fresh projection instead of mutating the shared nested one. This is OK, because we don't expect toString() to be used in the inner loop.

[dramaticlly]

This is similar to StructTransform::toString with values is already object array, so can probably simplify to

Suggested change

	return sb.toString();
	return Arrays.stream(values)
	.map(String::valueOf)
	.collect(Collectors.joining(", ", "[", "]"));

[anoopj]

Done. Thanks

missed a subtle subtle issue

[dramaticlly]

Thanks @anoopj!

[dramaticlly]

nit: would be great to have coverage on StructProjection.createAllowMissing(DATA_STRUCT_MISSING_NESTED_FIELD, PROJECTED_STRUCT) as well.

[anoopj]

Added toStringRendersMissingNestedFieldAsNull

[amogh-jahagirdar]

Thanks @anoopj and thank you @dramaticlly for the reviews!