parquet-tools merge extremely slow with block-option

parquet-tools merge is extremely time- and memory-consuming when used with block-option.

The merge function builds a bigger file out of several smaller parquet-files. Used without the block-option it just concatenates the files into a bigger one without building larger row-groups. That doesn't help with query-performance-issues. With block-option, parquet-tools build bigger row-groups which improves the query-performance, but the merge-process itself is extremely slow and memory-consuming.

Consider a case in which you have many small parquet files, e.g. 1000 files with a size of 100kb. Merging them into one file fails on my machine because even 20GB of memory are not enough for the process (the total amount of data as well as the resulting file should be smaller than 100MB).

Different situation: Consider having 100 files of size 1MB. Then merging them is possible with 20GB of RAM, but it takes almoust half an hour to process, which is to much for many use-cases.

Is there any possibility to accelerate the merge and reduce the need of memory?

**Reporter**: [Alexander Gunkel](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=Checkito)

_{**Note**: *This issue was originally created as [PARQUET-1670](https://issues.apache.org/jira/browse/PARQUET-1670). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

从 parquet-tools 的 merge 入口开始，复现报告的 block-option 场景：1,000 个 100 KB 文件和 100 个 1 MB 文件。比较使用和不使用 block-option 时的运行时间和内存；当使用 block-option 的 merge 在不过度使用内存的情况下完成，并且在保留更大的行组的同时显著更快时，即视为完成。

由索引模型根据 Issue 内容生成。