To develop more on point #1, even on a simple query like "select count(Time) from mytable where Time between '2018-11-27' and '2018-11-30'" with a short Time interval, the number of read files is consistently small and number of pruned files is consistently big.

So it makes me think that everything is starts by the Time filter, and small number of Time files implies small number of files with other columns. But the main question is HOW does it make that pruning? Maybe parquet tracks min/max value per file or something like that?