可以从Dask读取拼花地板元数据吗？

我有数千个拼花地板文件需要处理。在处理文件之前，我尝试使用拼花元数据获取有关文件的各种信息，例如每个分区中的行数、分钟数、最大值等

我尝试使用dask读取元数据。我希望在集群中分发元数据收集任务，但这似乎会导致dask不稳定。请参见下面的示例代码段和节点超时错误

有没有办法从Dask读取拼花地板元数据？我知道Dask的“read_parquet”函数有一个“gather_statistics”选项，您可以将其设置为false以加快文件读取速度。但是，如果设置为true，我看不到访问所有拼花地板元数据/统计信息的方法

示例代码：

@dask.delayed
def get_pf(item_to_read):
     pf = fastparquet.ParquetFile(item_to_read)
     row_groups = pf.row_groups.copy()
     all_stats = pf.statistics.copy()
     col = pf.info['columns'].copy()
     return [row_groups, all_stats, col]

stats_arr = get_pf(item_to_read)

示例错误：

2019-10-03 01:43:51,202 - INFO - 192.168.0.167 - distributed.worker - ERROR - Worker stream died during communication: tcp://192.168.0.223:34623

2019-10-03 01:43:51,203 - INFO - 192.168.0.167 - Traceback (most recent call last):

2019-10-03 01:43:51,204 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/distributed/comm/core.py", line 218, in connect

2019-10-03 01:43:51,206 - INFO - 192.168.0.167 -     quiet_exceptions=EnvironmentError,

2019-10-03 01:43:51,207 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/tornado/gen.py", line 729, in run

2019-10-03 01:43:51,210 - INFO - 192.168.0.167 -     value = future.result()

2019-10-03 01:43:51,211 - INFO - 192.168.0.167 - tornado.util.TimeoutError: Timeout

2019-10-03 01:43:51,212 - INFO - 192.168.0.167 -

2019-10-03 01:43:51,213 - INFO - 192.168.0.167 - During handling of the above exception, another exception occurred:

2019-10-03 01:43:51,214 - INFO - 192.168.0.167 -

2019-10-03 01:43:51,215 - INFO - 192.168.0.167 - Traceback (most recent call last):

2019-10-03 01:43:51,217 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/distributed/worker.py", line 1841, in gather_dep

2019-10-03 01:43:51,218 - INFO - 192.168.0.167 -     self.rpc, deps, worker, who=self.address

2019-10-03 01:43:51,219 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/tornado/gen.py", line 729, in run

2019-10-03 01:43:51,220 - INFO - 192.168.0.167 -     value = future.result()

2019-10-03 01:43:51,222 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/tornado/gen.py", line 736, in run

2019-10-03 01:43:51,223 - INFO - 192.168.0.167 -     yielded = self.gen.throw(*exc_info)  # type: ignore

2019-10-03 01:43:51,224 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/distributed/worker.py", line 3029, in get_data_from_worker

2019-10-03 01:43:51,225 - INFO - 192.168.0.167 -     comm = yield rpc.connect(worker)

2019-10-03 01:43:51,640 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/tornado/gen.py", line 729, in run

2019-10-03 01:43:51,641 - INFO - 192.168.0.167 -     value = future.result()

2019-10-03 01:43:51,643 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/tornado/gen.py", line 736, in run

2019-10-03 01:43:51,644 - INFO - 192.168.0.167 -     yielded = self.gen.throw(*exc_info)  # type: ignore

2019-10-03 01:43:51,645 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/distributed/core.py", line 866, in connect

2019-10-03 01:43:51,646 - INFO - 192.168.0.167 -     connection_args=self.connection_args,

2019-10-03 01:43:51,647 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/tornado/gen.py", line 729, in run

2019-10-03 01:43:51,649 - INFO - 192.168.0.167 -     value = future.result()

2019-10-03 01:43:51,650 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/tornado/gen.py", line 736, in run

2019-10-03 01:43:51,651 - INFO - 192.168.0.167 -     yielded = self.gen.throw(*exc_info)  # type: ignore

2019-10-03 01:43:51,652 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/distributed/comm/core.py", line 230, in connect

2019-10-03 01:43:51,653 - INFO - 192.168.0.167 -     _raise(error)

2019-10-03 01:43:51,654 - INFO - 192.168.0.167 -   File "/usr/local/lib/python3.7/dist-packages/distributed/comm/core.py", line 207, in _raise

2019-10-03 01:43:51,656 - INFO - 192.168.0.167 -     raise IOError(msg)

2019-10-03 01:43:51,657 - INFO - 192.168.0.167 - OSError: Timed out trying to connect to 'tcp://192.168.0.223:34623' after 10 s: connect() didn't finish in time

---

dd.read\u拼花地板花很长时间吗？如果没有，那么您可以按照其中的任何策略在客户机中进行阅读

如果数据在根目录中有一个
\u metadata
文件，那么您只需使用fastparquet打开它，这正是Dask所要做的。它包含所有数据块的所有详细信息

没有特别的理由认为分发元数据读取应该是一个问题，但是您应该知道，在某些情况下，总的元数据项加起来可能相当大。
谢谢，@mdurant。dd.read_parquet本身速度很快，直接给我提供了很多信息（例如列名），但是获取每个分区中的行数等信息要比直接读取元数据慢得多，因为您基本上必须持久化/计算整个dask读取操作。所以，这听起来像是延迟fastparquet元数据读取操作，并像我所做的那样计算这些操作来分发操作，这是最好的方法。也许我还有其他问题导致了不稳定。