Yuan-Ru-Lin commented on issue #434:
URL: https://github.com/apache/arrow-julia/issues/434#issuecomment-3093807424
> Is there a way to get the batch-offset table with Arrow.jl, if the data is
written in "file" mode?
Yes.
Consider `test.arrow` generated by the following script.
```julia
using Arrow
using TypedTables
using Tables
t = Table(
a=collect(1:10_000),
b=rand(Float32, 10_000),
c=rand(ComplexF32, 10_000),
)
# This would produce 10 RecordBatches
Arrow.write("test.arrow", Tables.partitioner(Iterators.partition(t, 1_000)))
```
Then one can get the indices of all the `RecordBatch`es by `read`ing the
relevant bytes and parsing them using
`Arrow.FlatBuffers.getrootas(Arrow.Meta.Footer, _footerbytes, 0)`
```julia
using Arrow
f = open("test.arrow")
# Check whether the magic number is there
seekend(f)
seek(f, position(f) - 6)
@assert String(read(f, 6)) == "ARROW1"
# Fetch footer size
seekend(f)
seek(f, position(f) - 6 - 4)
footersize = only(reinterpret(Int32, read(f, 4)))
@assert footersize == 560
# Fetch footer
seekend(f)
seek(f, position(f) - 6 - 4 - 560)
_footerbytes = read(f, 560)
_footer = Arrow.FlatBuffers.getrootas(Arrow.Meta.Footer, _footerbytes, 0)
"""
julia> _footer.recordBatches
10-element Arrow.FlatBuffers.Array{Arrow.Flatbuf.Block, NTuple{24, UInt8},
Arrow.Flatbuf.Footer}:
Arrow.Flatbuf.Block(offset = 320, metaDataLength = 320, bodyLength = 20000)
Arrow.Flatbuf.Block(offset = 20640, metaDataLength = 320, bodyLength =
20000)
Arrow.Flatbuf.Block(offset = 40960, metaDataLength = 320, bodyLength =
20000)
Arrow.Flatbuf.Block(offset = 61280, metaDataLength = 320, bodyLength =
20000)
Arrow.Flatbuf.Block(offset = 81600, metaDataLength = 320, bodyLength =
20000)
Arrow.Flatbuf.Block(offset = 101920, metaDataLength = 320, bodyLength =
20000)
Arrow.Flatbuf.Block(offset = 122240, metaDataLength = 320, bodyLength =
20000)
Arrow.Flatbuf.Block(offset = 142560, metaDataLength = 320, bodyLength =
20000)
Arrow.Flatbuf.Block(offset = 162880, metaDataLength = 320, bodyLength =
20000)
Arrow.Flatbuf.Block(offset = 183200, metaDataLength = 320, bodyLength =
20000)
"""
# Sanity check: fetch the first column in the first block using the above
information
seek(f, 320 + 320)
block1data = read(f, 20000)
reinterpret(Int64, block1data[1:8000])
"""
julia> reinterpret(Int64, block1data[1:8000])
1000-element reinterpret(Int64, ::Vector{UInt8}):
1
2
3
4
(omitted)
"""
```
I accessed the first batch but in principle one can access to whichever
block without reading others.
In order to come up with an API, I still need to know how to parse bytes
that make up a `RecordBatch`.
By the way, this might provide a way to closing
https://github.com/apache/arrow-julia/issues/353
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