aokolnychyi commented on a change in pull request #1499:
URL: https://github.com/apache/iceberg/pull/1499#discussion_r495610870
##########
File path: site/docs/spec.md
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@@ -138,19 +164,41 @@ Valid type promotions are:
* `float` to `double`
* `decimal(P, S)` to `decimal(P', S)` if `P' > P` -- widen the precision of
decimal types.
-Any struct, including a top-level schema, can evolve through deleting fields,
adding new fields, renaming existing fields, or promoting a primitive using the
valid type promotions. Adding a new field assigns a new ID for that field and
for any nested fields. Renaming an existing field must change the name, but not
the field ID. Deleting a field removes it from the current schema. Field
deletion cannot be rolled back unless the field was nullable or if the current
snapshot has not changed.
+Any struct, including a top-level schema, can evolve through deleting fields,
adding new fields, renaming existing fields, reordering existing fields, or
promoting a primitive using the valid type promotions. Adding a new field
assigns a new ID for that field and for any nested fields. Renaming an existing
field must change the name, but not the field ID. Deleting a field removes it
from the current schema. Field deletion cannot be rolled back unless the field
was nullable or if the current snapshot has not changed.
Grouping a subset of a struct’s fields into a nested struct is **not**
allowed, nor is moving fields from a nested struct into its immediate parent
struct (`struct<a, b, c> ↔ struct<a, struct<b, c>>`). Evolving primitive types
to structs is **not** allowed, nor is evolving a single-field struct to a
primitive (`map<string, int> ↔ map<string, struct<int>>`).
+#### Column Projection
+
+Columns in Iceberg data files are selected by field id. Column names and order
may change between after a data file is written, and projection must be done
using field ids. If a field id is missing from a data file, its value for each
row should be `null`.
+
+For example, a file may be written with schema `1: a int, 2: b string, 3: c
double` and read using projection schema `3: measurement, 2: name, 4: a`. This
must select file columns `c` (renamed to `measurement`), `b` (now called
`name`), and a column of `null` values called `a`; in that order.
+
+
+#### Reserved Field IDs
+
+Iceberg tables must not use field ids greater than 2147483447
(`Integer.MAX_VALUE - 200`). This id range is reserved for metadata columns
that can be used in user data schemas, like the `_file` column that holds the
file path in which a row was stored.
+
+The set of metadata columns is:
+
+| Field id, name | Type | Description |
+|-----------------------------|---------------|-------------|
+| **`2147483646 _file`** | `string` | Path of the file in which a
row is stored |
+| **`2147483645 _pos`** | `long` | Ordinal position of a row in
the source data file |
+| **`2147483546 file_path`** | `string` | Path of a file, used in
position-based delete files |
+| **`2147483545 pos`** | `long` | Ordinal position of a row,
used in position-based delete files |
+| **`2147483544 row`** | `struct<...>` | Deleted row values, used in
position-based delete files |
Review comment:
equality?
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