Issue: `ArrowInvalid: Added column's length must match table's length.` after using `select`
State: closed
Author: lvwerra

## Describe the bug
I would like to add a column to a downsampled dataset. However I get an error message saying the length don't match with the length of the unsampled dataset indicated. I suspect that the dataset size is not updated when calling `select`.

## Steps to reproduce the bug
```python
from datasets import load_dataset

ds = load_dataset("tweets_hate_speech_detection")['train'].select(range(128))
ds = ds.add_column('ones', [1]*128)
```

## Expected results
I would expect a new column named `ones` filled with `1`. When I check the length of `ds` it says `128`. Interestingly, it works when calling `ds = ds.map(lambda x: x)` before adding the column.

## Actual results
Specify the actual results or traceback.
```python
---------------------------------------------------------------------------
ArrowInvalid                              Traceback (most recent call last)
/var/folders/l4/2905jygx4tx5jv8_kn03vxsw0000gn/T/ipykernel_6301/868709636.py in <module>
      1 from datasets import load_dataset
      2 ds = load_dataset("tweets_hate_speech_detection")['train'].select(range(128))
----> 3 ds = ds.add_column('ones', [0]*128)

~/git/semantic-clustering/env/lib/python3.8/site-packages/datasets/arrow_dataset.py in wrapper(*args, **kwargs)
    183         }
    184         # apply actual function
--> 185         out: Union["Dataset", "DatasetDict"] = func(self, *args, **kwargs)
    186         datasets: List["Dataset"] = list(out.values()) if isinstance(out, dict) else [out]
    187         # re-apply format to the output

~/git/semantic-clustering/env/lib/python3.8/site-packages/datasets/fingerprint.py in wrapper(*args, **kwargs)
    395             # Call actual function
    396 
--> 397             out = func(self, *args, **kwargs)
    398 
    399             # Update fingerprint of in-place transforms + update in-place history of transforms

~/git/semantic-clustering/env/lib/python3.8/site-packages/datasets/arrow_dataset.py in add_column(self, name, column, new_fingerprint)
   2965         column_table = InMemoryTable.from_pydict({name: column})
   2966         # Concatenate tables horizontally
-> 2967         table = ConcatenationTable.from_tables([self._data, column_table], axis=1)
   2968         # Update features
   2969         info = self.info.copy()

~/git/semantic-clustering/env/lib/python3.8/site-packages/datasets/table.py in from_tables(cls, tables, axis)
    715             table_blocks = to_blocks(table)
    716             blocks = _extend_blocks(blocks, table_blocks, axis=axis)
--> 717         return cls.from_blocks(blocks)
    718 
    719     @property

~/git/semantic-clustering/env/lib/python3.8/site-packages/datasets/table.py in from_blocks(cls, blocks)
    663             return cls(table, blocks)
    664         else:
--> 665             table = cls._concat_blocks_horizontally_and_vertically(blocks)
    666             return cls(table, blocks)
    667 

~/git/semantic-clustering/env/lib/python3.8/site-packages/datasets/table.py in _concat_blocks_horizontally_and_vertically(cls, blocks)
    623             if not tables:
    624                 continue
--> 625             pa_table_horizontally_concatenated = cls._concat_blocks(tables, axis=1)
    626             pa_tables_to_concat_vertically.append(pa_table_horizontally_concatenated)
    627         return cls._concat_blocks(pa_tables_to_concat_vertically, axis=0)

~/git/semantic-clustering/env/lib/python3.8/site-packages/datasets/table.py in _concat_blocks(blocks, axis)
    612                 else:
    613                     for name, col in zip(table.column_names, table.columns):
--> 614                         pa_table = pa_table.append_column(name, col)
    615             return pa_table
    616         else:

~/git/semantic-clustering/env/lib/python3.8/site-packages/pyarrow/table.pxi in pyarrow.lib.Table.append_column()

~/git/semantic-clustering/env/lib/python3.8/site-packages/pyarrow/table.pxi in pyarrow.lib.Table.add_column()

~/git/semantic-clustering/env/lib/python3.8/site-packages/pyarrow/error.pxi in pyarrow.lib.pyarrow_internal_check_status()

~/git/semantic-clustering/env/lib/python3.8/site-packages/pyarrow/error.pxi in pyarrow.lib.check_status()

ArrowInvalid: Added column's length must match table's length. Expected length 31962 but got length 128
```

## Environment info
- `datasets` version: 1.11.0
- Platform: macOS-10.16-x86_64-i386-64bit
- Python version: 3.8.5
- PyArrow version: 5.0.0


Comments:
Hi,

the `select` method creates an indices mapping and doesn't modify the underlying PyArrow table by default for better performance. To modify the underlying table after the `select` call, call `flatten_indices` on the dataset object as follows:
```python
from datasets import load_dataset

ds = load_dataset("tweets_hate_speech_detection")['train'].select(range(128))
ds = ds.flatten_indices()
ds = ds.add_column('ones', [1]*128)
```

Thanks for the question @lvwerra. And thanks for the answer @mariosasko. ^^