When importing CSV/TSV to mongodb, the option --columnsHaveTypes can help to define the columnstypes. But the document seems very unclear. I tried several times until finally did succeed.
You should add option --columnsHaveTypes and change every column after --fields and remember using "" before "(" and ")".
for example, change:
mongoimport -h foohost -d bardb -c fooc --type tsv --fields col1,col2,col3 --file path/to/file.txt
into
mongoimport -h foohost -d bardb -c fooc --type tsv --fields col1.int32(),col2.double(),col3.string() --columnsHaveTypes --file path/to/file.txt
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