nastra commented on code in PR #14234:
URL: https://github.com/apache/iceberg/pull/14234#discussion_r3207678943
##########
format/spec.md:
##########
@@ -707,6 +714,131 @@ For `geography` only, xmin (X value of `lower_bounds`)
may be greater than xmax
When calculating upper and lower bounds for `geometry` and `geography`, null
or NaN values in a coordinate dimension are skipped; for example, POINT (1 NaN)
contributes a value to X but no values to Y, Z, or M dimension bounds. If a
dimension has only null or NaN values, that dimension is omitted from the
bounding box. If either the X or Y dimension is missing then the bounding box
itself is not produced.
+##### Content Stats
+
+In Iceberg v4 stats have been redesigned and are represented by using nested
structs (`struct<struct<...>>`). The statistics for fields are tracked inside a
nested struct of value counts and bounds (described in the next section). Each
field-level statistics struct is a field of the `content_stats` struct, which
holds all statistics for table fields.
+
+###### ID assignment for stats fields
+
+ID assignment follows a deterministic mapping from the **table ID space** to
the **stats ID space**, where a given field ID from the **table ID space** gets
an ID assigned from the **stats ID space** for each field-level statistics
struct.
+Each field-level statistic listed in the [field stats types
section](#field-stats-types) has a fixed offset. Its stats field ID is the
enclosing stats struct's ID plus that offset.
+
+**Data columns (normal table field ids)**
+Mapping a table field ID from the **table ID space** to the **stats ID space**
is done via:
+
+`stats_struct_id = 10_000 + (200 * table_field_id)`
+
+The constant `10_000` is `stats_space_field_id_start_for_data_fields`. `200`
represents the number of supports stats per column
(`num_supported_stats_per_column = 200`).
+
+The formula is defined as:
+`stats_struct_id = stats_space_field_id_start_for_data_fields +
(num_supported_stats_per_column * table_field_id)`
+
+Each field statistic listed under [Field stats types](#field-stats-types) has
a fixed **offset** within that block. The field id for an individual field
statistic is:
+
+`stats_field_id = stats_struct_id + offset`
+
+**Metadata columns (reserved table field ids)**
+
+[Reserved metadata fields](#reserved-field-ids) use a different starting base
for their stats field ids in order to not overlap with data field stats ids.
Mapping a reserved table field ID to the **stats ID space** is done via:
+
+`stats_struct_id = 2_147_000_000 + (200 * (200 - (Integer.MAX_VALUE -
table_field_id)))`
+
+Here `2_147_000_000` is `stats_space_field_id_start_for_metadata_fields`. This
separate base is required because reserved ids are near `Integer.MAX_VALUE` and
cannot use the same linear mapping as data field ids.
+The first `200` refers to `num_supported_stats_per_column = 200` and the
second `200` refers to `num_reserved_field_ids = 200` from [Reserved field
ids](#reserved-field-ids).
+
+The formula is defined as:
+`stats_struct_id = stats_space_field_id_start_for_metadata_fields +
(num_supported_stats_per_column * (num_reserved_field_ids - (Integer.MAX_VALUE
- table_field_id)))`
+
+Valid data field ids support stats structs with ids from `10_000` through
`200_010_000`, so the highest supported **data** field id is `1_000_000`.
+
+###### Name assignment for `content_stats` fields
+
+Each nested stats struct is a **child field** of the root `content_stats`
struct. Its **name** is the numerical string of the table column's field id
(for example id `103` uses the name `"103"`).
+Its **field id** is deterministically calculated as defined in the previous
section. The name is informational and readers must resolve content stats by ID.
+
+###### Field stats types
+
+Each stats struct holds statistics for one table column. It may contain the
following metrics:
+
+| required/optional | Offset | Name | Type |
included for | Description
|
+|-------------------|--------|-------------------------|---------------------|-------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| _optional_ | 1 | value_count | `long` |
all types | Number of values in the column (including null and
NaN values)
|
Review Comment:
I think we might want to leave out avg/max value sizes in this case then,
because we wouldn't be using those right away, unless I'm missing where else
we'd be immediately using the avg value size?
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