# Reading multiple matrix tables

**URL:** <https://discuss.hail.is/t/reading-multiple-matrix-tables/1212>\
**Category:** Hail Query & hailctl\
**Created:** [December 11, 2019, 11:33am UTC](https://discuss.hail.is/t/reading-multiple-matrix-tables/1212 "2019-12-11T11:33:47Z")\
**Posts on this page:** 1\
**Showing post:** 2

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**Author:** ![tpoterba](https://yyz2.discourse-cdn.com/flex036/user_avatar/discuss.hail.is/tpoterba/32/61_2.png) [@tpoterba](https://discuss.hail.is/u/tpoterba)\
**Post date:** [December 11, 2019, 12:37pm UTC](https://discuss.hail.is/t/reading-multiple-matrix-tables/1212/2 "2019-12-11T12:37:45Z")

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this is intentional; a MatrixTable is already a composite object, so it shouldn’t be a common use case to glob them.

You can just iterate in Python. First, let’s define a helper function that makes a nested union N log N, not quadratic (see [here](https://discuss.hail.is/t/performance-of-writing-matrixtable-on-0-2/663/9) for more info):

```python
def union_cols_all(mts):
    mts = mts[:]

    iteration = 0
    while (len(mts) > 1):
        iteration += 1
        print(f'iteration {iteration}')
        tmp = []
        for i in range(0, len(mts), 2):
            tmp.append(mts[i].union_cols(mts[i+1]))
        mts = tmp[:]
    return mts[0]

```

And then read and union in Python:

```python
files = [f'/path/to/chr{chrom}' for chrom in list(range(23)) + ['X', 'Y']]
mt = union_cols_all([hl.read_matrix_table(file) for file in files])

```

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