emkornfield commented on code in PR #542:
URL: https://github.com/apache/parquet-format/pull/542#discussion_r2660269228
##########
src/main/thrift/parquet.thrift:
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@@ -958,6 +958,21 @@ union ColumnCryptoMetaData {
struct ColumnChunk {
/** File where column data is stored. If not set, assumed to be same file as
* metadata. This path is relative to the current file.
+ *
+ * As of December 2025, the only known use-case for this field is writing
summary
+ * parquet files (i.e. "_metadata" files). These files consolidate footers
from
+ * multiple parquet files to allow for efficient reading of footers to
avoid file
+ * listing costs and prune out files that do not need to be read based on
statistics.
+ *
+ * These files do not appear to have ever been formally specified in the
specification.
+ * and are potentially problematic from a correctness perspective [1].
+ *
+ * [1] https://lists.apache.org/thread/ootf2kmyg3p01b1bvplpvp4ftd1bt72d
+ *
+ * There is no other known usage of this field. Specifically, there are no
known
+ * readers that will read externally stored column data if this field is
populated
Review Comment:
```suggestion
* reference implementations that will read externally stored column data
if this field is populated
```
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