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https://issues.apache.org/jira/browse/PARQUET-1670?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16941886#comment-16941886
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Gabor Szadovszky commented on PARQUET-1670:
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OK, I forgot about it. I was the one who reverted this feature. It was trying
to do some more advanced merging but the concept was not correct. I would not
suggest using it.
> parquet-tools merge extremely slow with block-option
> ----------------------------------------------------
>
> Key: PARQUET-1670
> URL: https://issues.apache.org/jira/browse/PARQUET-1670
> Project: Parquet
> Issue Type: Bug
> Reporter: Alexander Gunkel
> Priority: Major
>
> 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?
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