stevenzwu commented on code in PR #18585:
URL: https://github.com/apache/iceberg/pull/18585#discussion_r4340444450
##########
format/spec.md:
##########
@@ -1008,15 +1007,30 @@ 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.
+###### Stats Tightness
+
+Statistics are _tight_ when the bounds, `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.
+* `null_value_count` and `nan_value_count` are the counts across the live rows.
+
+Bounds for `string` and `binary` are never tight because they may be truncated
when written. Bounds for `geometry` and `geography` are a bounding box, not
exact minimum and maximum values.
+
+`variant` bounds are stored per leaf field and are optional; a leaf field may
have no bound, and each present bound follows the rules above for its type. 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.
Review Comment:
> 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.
This sentence defines what `null_value_count` includes for variant type, so
it doesn't belong in the tightness section. If we want to clarify it, it fits
with the `null_value_count` definition in [Field-level Metrics and
Statistics](https://iceberg.apache.org/spec/#field-level-metrics-and-statistics),
next to [Bounds for
Variant](https://iceberg.apache.org/spec/#bounds-for-variant).
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