reading of parquet binary type, is very slow:

[ldn9638]

I find [ https://github.com/jorgecarleitao/arrow2/blob/main/src/io/parquet/read/binary/mod.rs#L34]
capacity cannot be pre allocated here，Cause ‘extend_from_Slice’ is very slow。

[jorgecarleitao]

Ahahaha I also found that out yesterday! Fully agree, we should pre-reserve this one for sure. Not that in plain, a value is always [4 bytes][values] and each page anounces how many values it contains, so we can pre-allocate exactly.