pkg/query: Check individual mapping fields for null

[metalmatze]

It seems like mappingStart was valid (not null) but the build ID index was.

[alwaysmeticulous]

✅ Meticulous spotted zero visual differences across 406 screens tested: view results.

_{Last updated for commit 4043468. This comment will update as new commits are pushed.}

[brancz]

Can we try to understand why this happens? As far as I can tell all codepaths that generate these inputs write a non-null value for all mapping fields if writing a mapping start, so I don't understand why this happens in the first place.

[brancz]

As in, this is the code that produces this:

parca/pkg/parcacol/arrow.go

Lines 338 to 354 in 00471e8

	if loc.Mapping != nil {
	w.MappingStart.Append(loc.Mapping.Start)
	w.MappingLimit.Append(loc.Mapping.Limit)
	w.MappingOffset.Append(loc.Mapping.Offset)
	if err := w.MappingFile.Append([]byte(loc.Mapping.File)); err != nil {
	return nil, fmt.Errorf("append mapping file: %w", err)
	}
	if err := w.MappingBuildID.Append([]byte(loc.Mapping.BuildId)); err != nil {
	return nil, fmt.Errorf("append mapping build id: %w", err)
	}
	} else {
	w.MappingStart.AppendNull()
	w.MappingLimit.AppendNull()
	w.MappingOffset.AppendNull()
	w.MappingFile.AppendNull()
	w.MappingBuildID.AppendNull()
	}

[metalmatze]

Really odd indeed. Reading around the code you posted I briefly assumed that we might be hitting this part:
https://github.com/apache/arrow/blob/50015f084846faaa5bf867346d045499e3f68316/go/arrow/array/dictionary.go#L1300-L1303

However, we are calling Append([]byte(loc.Mapping.BuildId)). loc.Mapping cannot be nil as we check that right above and even if loc.Mapping.BuildId == "" the dictionary won't appendNull, as "" != nil. 🤔

[brancz]

Is there maybe something special about the conversion of an empty string?