apache / apache/parquet-java

null_count is omitted for large columns in parquet files

Open Beginner friendly
#3,574 1 comment 0 reactions 0 assignees View on GitHub
Type: bug
Dominant language
Java
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Description

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

Currently in [ParquetMetadataConverter.java](https://github.com/apache/parquet-java/blob/7be05b4702df78ae0c0c6b44adc6b7b7af2d931f/parquet-hadoop/src/main/java/org/apache/parquet/format/converter/ParquetMetadataConverter.java), there is a guard that prevents the writing of statistics such as min/max AND null_count when the stats are larger than the max allowed size under truncation. The rationale for this makes sense for omitting min/max, however null_count can be written on the file despite the size of its content. See the code below:

https://github.com/apache/parquet-java/blob/7be05b4702df78ae0c0c6b44adc6b7b7af2d931f/parquet-hadoop/src/main/java/org/apache/parquet/format/converter/ParquetMetadataConverter.java#L800-L807

The missing `null_count` metadata sometimes causes downstream consumers of the parquet files to error. For example in Snowflake we are seeing the following kind of error:

```
non-nullable column without default has null values according to file statistics
```

### Component(s)

Core

Contributor guide

No contributing guide indexed for this repository

Research direction

Start in parquet-hadoop/src/main/java/org/apache/parquet/format/converter/ParquetMetadataConverter.java at the guard around lines 800-807. Check how the size limit currently suppresses statistics, then ensure null_count is retained even when min/max are omitted; done means large-column Parquet files include null_count metadata and avoid the reported downstream error.

Written by the indexing model from the issue text.

Assessment

Tech stack
java
Domain
data-engineering
Issue type
Bug
Difficulty
2/5
Estimated time
1-3 hours
Activity status
Active
Clarity
Clearly specified
Newbie friendliness
78/100

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