jorisvandenbossche commented on code in PR #43950:
URL: https://github.com/apache/arrow/pull/43950#discussion_r1751922993
##########
docs/source/format/Columnar.rst:
##########
@@ -1385,6 +1387,59 @@ have two entries in each RecordBatch. For a RecordBatch
of this schema with
buffer 13: col2 data
+Compression
+-----------
+
+There are three different options for compression of record batch
+body buffers: Buffers can be uncompressed, buffers can be
+compressed with the ``lz4`` compression codec, or buffers can
+be compressed with the ``zstd`` compression codec. Buffers in
+the flat sequence of a message body must be either all
+uncompressed or all compressed separately using the same codec.
+
+.. note::
+
+ ``lz4`` compression codec means the
+ `LZ4 frame format
<https://github.com/lz4/lz4/blob/dev/doc/lz4_Frame_format.md>`_
+ and should not to be confused with
+ `"raw" (also called "block") format
<https://github.com/lz4/lz4/blob/dev/doc/lz4_Block_format.md>`_.
+
+The difference between compressed and uncompressed buffers in the
+serialized form is as follows:
+
+* If the buffers in the :ref:`ipc-recordbatch-message` are **compressed**
+
+ - the ``data header`` includes the length and memory offset
+ of each **compressed buffer** in the record batch's body
+
+ - the ``body`` includes a flat sequence of **compressed buffers**
+ together with the **length of uncompressed buffer** as a 64-bit
+ little-endian signed integer stored in the first 8 bytes for each
+ buffer in the sequence
+
+* If the buffers in the :ref:`ipc-recordbatch-message` are **uncompressed**
+
+ - the ``data header`` includes the length and memory offset
+ of each **uncompressed buffer** in the record batch's body
+
+ - the ``body`` includes a flat sequence of **uncompressed buffers**
+ with the first 8 bytes empty or equal to ``-1`` to indicate that
+ the buffer is uncompressed
Review Comment:
I think this is not correct? If the the buffers are **un**compressed in
general (i.e. indicated as such in the header), then there is no first 8 bytes
indicating -1. That is only the case for a single buffer that is not compressed
_when_ the buffers are expected to all be compressed.
(that's also the reason I was asking above in the bullet point about
compressed data if it shouldn't be added)
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