apache / apache/parquet-java

parquet-cli reports nested columns as null

Open
#3,095 0 comments 0 reactions 0 assignees View on GitHub
Type: bug
Dominant language
Java
Stars
3.1k
Forks
1.6k
Avg merge
3d 12h
Merged PRs (30d)
33

Description

### Describe the bug, including details regarding any error messages, version, and platform.

Using Parquet CLI 1.15.0 via Mac Homebrew, I noticed some surprising behaviour with the `parquet-cli` and nested columns.

`parquet schema catalog.parquet` returns a schema showing the nested types (I've trimmed the field list slightly):

```avro
{
"type" : "record",
"name" : "schema",
"fields" : [ {
"name" : "item_id",
"type" : "string"
}, {
"name" : "title",
"type" : [ "null", "string" ],
"default" : null
}, {
"name" : "language",
"type" : [ "null", "string" ],
"default" : null
}, {
"name" : "subjects",
"type" : [ "null", {
"type" : "array",
"items" : {
"type" : "record",
"name" : "list",
"fields" : [ {
"name" : "element",
"type" : "string"
} ]
}
} ],
"default" : null
}, {
"name" : "authors",
"type" : [ "null", {
"type" : "array",
"items" : {
"type" : "record",
"name" : "list",
"namespace" : "list2",
"fields" : [ {
"name" : "element",
"type" : "string"
} ]
}
} ],
"default" : null
} ]
}
```

`parquet dictionary -c subjects.list.element catalog.parquet` will return the expected values for those fields as well:

```
Row group 0 dictionary for "subjects.list.element":
0: "Bestsellers"
1: "Biography"
2: "Fantasy Fiction"
3: "Music Theory"
4: "Disability"
5: "Family"
6: "Young Adult"
```

However, when using `cat` or `head` to display the file contents those fields are displayed as null:

```
{"bmc_id": "id1", "title": null, "language": "en", "subjects": null, "authors": null}
{"bmc_id": "id2", "title": null, "language": "en", "subjects": null, "authors": null,}
{"bmc_id": "id3", "title": null, "language": "en", "subjects": null, "authors": null}
```

Other tools like PyArrow or Pandas do display those values as arrays. I created this as a bug because it _looks_ like it's working and if those fields are nullable, there's no way to tell whether the null value is correct.

### Component(s)

_No response_

Contributor guide

No contributing guide indexed for this repository

Assessment

This issue has not been assessed yet.

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.