stevenzwu commented on code in PR #18585:
URL: https://github.com/apache/iceberg/pull/18585#discussion_r4343729259
##########
format/spec.md:
##########
@@ -1008,15 +1007,32 @@ For example, stats for an optional `geometry` field
named `location` with field-
10_816: optional double z;
10_817: optional double m;
}
- 10_804: optional long value_count;
- 10_805: optional long null_value_count;
- 10_807: optional long total_bytes;
- // tight_bounds and nan_value_count are omitted for geo types
+ 10_803: optional long value_count;
+ 10_804: optional long null_value_count;
+ 10_806: optional long total_bytes;
+ // nan_value_count is omitted for geo types
}
```
For `variant`, both bounds are unshredded `variant` that store variant field
bounds by normalized JSON paths as field names. See [Bounds for
Variant](#bounds-for-variant) for details on producing these bounds.
+A `variant` value is null only when the variant itself is null; a non-null
variant that encodes a variant null is not counted as a null value.
+
+###### Stats Tightness
+
+Statistics are _tight_ when the bounds, `value_count`, `null_value_count`, and
`nan_value_count` are exact for the live rows of a file:
+
+* `lower_bound` and `upper_bound` for fixed-length types are equal to the
minimum and maximum of the live values.
+* `value_count`, `null_value_count`, and `nan_value_count` are the counts
across the live rows.
+
+Regardless of `tight_stats`, bounds for `string` and `binary` are never tight
because they may be truncated when written, and bounds for `geometry` and
`geography` are never tight because they are a bounding box rather than exact
minimum and maximum values.
+
+For `variant`, `lower_bound` and `upper_bound` hold one bound per normalized
JSON path. A path may be omitted, which means that path has no bound. Whether a
present bound is tight follows the same rules as above for that path's value
type.
+
+Statistics other than the bounds, `value_count`, `null_value_count`, and
`nan_value_count` are not part of tightness.
+
+When `tight_stats` is absent, readers must assume stats are not tight;
`tight_stats` is the only indicator of tightness. Stats may be recomputed to
restore tightness without rewriting the data file.
Review Comment:
> ; `tight_stats` is the only indicator of tightness
this sentence is probably not needed
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