rdblue commented on code in PR #14234:
URL: https://github.com/apache/iceberg/pull/14234#discussion_r3230359565
##########
format/spec.md:
##########
@@ -704,11 +727,111 @@ Examples of valid field paths using normalized JSON path
format are:
* `$['tags']` -- the `tags` array
* `$['addresses']['zip']` -- the `zip` field in an `addresses` array that
contains objects
-For `geometry` and `geography` types, `lower_bounds` and `upper_bounds` are
both points of the following coordinates X, Y, Z, and M (see Appendix G) which
are the lower / upper bound of all objects in the file.
+##### Content Stats
-For `geography` only, xmin (X value of `lower_bounds`) may be greater than
xmax (X value of `upper_bounds`), in which case an object in this bounding box
may match if it contains an X such that x >= xmin OR x <= xmax. In geographic
terminology, the concepts of xmin, xmax, ymin, and ymax are also known as
westernmost, easternmost, southernmost and northernmost, respectively. These
points are further restricted to the canonical ranges of [-180..180] for X and
[-90..90] for Y.
+In Iceberg v4, statistics are stored in typed fields grouped in a struct that
corresponds to the table field. These stats structs are nested within the
`content_stats` struct in manifest files.
-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.
+###### Field Statistics
+
+Field-level structs in `content_stats` are based on the corresponding table
field's type, requirement, and ID (`field-id`).
+
+Field stats structs are assigned a range of 200 IDs, starting at `10_000 + 200
* field-id`. The first ID in the range (`base-id`) is the ID of the struct
field in `content_stats`. Fields within the stats struct are assigned IDs from
the range by adding an offset to the `base-id`. For example, the stats struct
for table field 2 uses IDs in the range `[10_400, 10_599]`, the field within
`content_stats` has uses the `base-id`, ID `10_400`, and its `lower_bound`
field (offset 1) uses ID `10_401`.
+
+Content stats must be resovled by ID; field names used for stats structs are
informational. The recommended name for each field is the full name of the
field in the table schema.
+
+IDs in the range `10_000` (inclusive) to `200_000_000` (exclusive) are
reserved for column stats structs in `content_stats`. Stats for table fields
with IDs outside that range cannot be stored in `content_stats`.
+
+[Reserved metadata fields](#reserved-field-ids) must use the stats ID ranges
from the following table. Stats for metadata fields not in the table are not
tracked.
+
+| Reserved field | ID | `base-id` | Range end |
+|---------------------------------|------------|-----------|-----------|
+| `_last_updated_sequence_number` | 2147483539 | 9000 | 9199 |
+| `_row_id` | 2147483540 | 9200 | 9399 |
+
+Each stats struct holds statistics for one table field. It may contain the
following metrics:
+
+| Requirement | Offset | Name | Type
| Included for | Description |
+|-------------|--------|---------------------------|---------------------------|-----------------------------------------------|-------------|
+| _optional_ | 1 | `lower_bound` | Field type or `geo_lower`
| all primitives or `variant` | Lower bound stored as the
field's type, or `geo_lower` for geo types |
+| _optional_ | 2 | `upper_bound` | Field type or `geo_upper`
| all primitives or `variant` | Upper bound stored as the
field's type, or `geo_upper` for geo types |
+| _optional_ | 3 | `tight_bounds` | `boolean`
| all except `geometry`, `geography`, `variant` | When true, `lower_bound` and
`upper_bound` must be equal to the min and max values |
+| _optional_ | 4 | `value_count` | `long`
| all | Number of values in the
column (including null and NaN values) |
+| _optional_ | 5 | `null_value_count` | `long`
| optional fields | Number of null values in the
column |
+| _optional_ | 6 | `nan_value_count` | `long`
| `float`, `double` | Number of NaN values in the
column |
+| _optional_ | 7 | `avg_value_size_in_bytes` | `int`
| `string`, `binary`, `variant` | Avg value size (uncompressed)
in bytes to estimate memory consumption |
+
+For example, stats for a `required` `int` field named `id` with field-id `2`
are stored using:
+
+```
+10_400: optional struct id (default null) {
+ 10_401: optional int lower_bound; // type matches the field type (int)
+ 10_402: optional int upper_bound; // type matches the field type (int)
+ 10_403: optional boolean tight_bounds;
+ 10_404: optional long value_count;
+
+ // null_value_count is only used for optional fields
+ // nan_value_count is only used for float and double
+ // avg_value_size_in_bytes is only used for variable length types
+}
+```
+
+If any field is missing from the struct, readers must assume that it is
unknown.
+
+Lower and upper bounds for `geometry` and `geography` columns are XYZM points
that define a bounding box, stored in `geo_lower` and `geo_upper` structs (see
[Bounds for Geometry and Geography](#bounds-for-geometry-and-geography). IDs
used by geo structs are assigned using offsets in the table field's stats ID
range.
+
+The `geo_lower` struct is defined as:
+
+| Requirement | Offset | Name | Type | Description |
+|-------------|--------|------|----------|-------------|
+| _required_ | 10 | `x` | `double` | Bounding box westernmost/xmin;
[-180..180] |
Review Comment:
All of the offsets are reserved if they aren't specified here. I just wanted
to give the geo bound structs memorable offsets.
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