ianmcook commented on code in PR #43950:
URL: https://github.com/apache/arrow/pull/43950#discussion_r1743745123
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docs/source/format/Columnar.rst:
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@@ -1385,6 +1385,36 @@ have two entries in each RecordBatch. For a RecordBatch
of this schema with
buffer 13: col2 data
+Compression
+-----------
+
+There are three different options for record batch body
+buffers compression: buffers can be uncompressed, can use
+``lz4`` or ``zstd`` compression codec. All buffers in the flat
+sequence of the message body are compressed separately with the
+same codec.
+
+The difference between compressed and uncompressed buffers in the
+serialized form is as follows:
+
+* If the buffers in the ``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 memory
+ buffers** together with the **length of uncompressed buffer**
+ stored in the first 8 bytes for each buffer in the sequence
+
+* If the buffers in the ``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 memory
+ buffers** with the first 8 bytes empty or equal to ``-1`` to
+ indicate that the buffer is uncompressed
Review Comment:
Small suggestion: consistently use the phrase "buffer" or "memory buffer"
instead of mixing and matching
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