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.*}

Beginnen Sie am parquet-tools-Merge-Einstiegspunkt und reproduzieren Sie die gemeldeten block-option-Fälle: 1,000 Dateien mit 100 KB und 100 Dateien mit 1 MB. Vergleichen Sie Laufzeit und Speicherverbrauch mit und ohne block-option; abgeschlossen ist die Aufgabe, wenn der block-option-Merge ohne übermäßigen Speicherverbrauch abgeschlossen wird und bei gleichzeitig größeren Row-Groups deutlich schneller ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.