import datachain as dc

x = dc.read_records(
    [ {'idx': i} for i in range(10000) ],
    schema={'idx': int}
)
y = dc.read_records(
    [ {'idx': i} for i in range(10000, 20000) ],
    schema={'idx': int}
)

z = x | y
print('before:', x.count(), y.count(), z.count())

z = z.persist()
print('after:', x.count(), y.count(), z.count())