jayzhan211 commented on code in PR #14362:
URL: https://github.com/apache/datafusion/pull/14362#discussion_r1946315799
##########
datafusion/common/src/dfschema.rs:
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@@ -1056,6 +1079,107 @@ pub fn qualified_name(qualifier:
Option<&TableReference>, name: &str) -> String
}
}
+/// Extension trait to manage DataFusion specific metadata on Arrow fields.
+pub trait FieldExt {
+ /// Check if this field is a system columns.
+ ///
+ /// System columns are columns which meant to be semi-public stores of the
internal details of the table.
+ /// For example, `ctid` in Postgres would be considered a metadata column
+ /// (Postgres calls these "system columns", see [the Postgres
docs](https://www.postgresql.org/docs/current/ddl-system-columns.html) for more
information and examples.
+ /// Spark has a `_metadata` column that it uses to include details about
each file read in a query (see [Spark's
docs](https://docs.databricks.com/en/ingestion/file-metadata-column.html)).
+ ///
+ /// DataFusion allows fields to be declared as metadata columns by setting
the `datafusion.system_column` key in the field's metadata
+ /// to `true`.
+ ///
+ /// As an example of how this works in practice, if you have the following
Postgres table:
+ ///
+ /// ```sql
+ /// CREATE TABLE t (x int);
+ /// INSERT INTO t VALUES (1);
+ /// ```
+ ///
+ /// And you do a `SELECT * FROM t`, you would get the following schema:
+ ///
+ /// ```text
+ /// +---+
+ /// | x |
+ /// +---+
+ /// | 1 |
+ /// +---+
+ /// ```
+ ///
+ /// But if you do `SELECT ctid, * FROM t`, you would get the following
schema (ignore the meaning of the value of `ctid`, this is just an example):
+ ///
+ /// ```text
+ /// +-----+---+
+ /// | ctid| x |
+ /// +-----+---+
+ /// | 0 | 1 |
+ /// +-----+---+
+ /// ```
+ fn is_system_column(&self) -> bool;
+
+ /// Mark this field as a system column.
+ ///
+ /// See [`FieldExt::is_system_column`] for more information on what a
system column is.
+ fn to_system_column(self) -> Self;
+
+ /// Mark this field as a non system column by removing the
`datafusion.system_column` key from the field's metadata.
+ ///
+ /// See [`FieldExt::is_system_column`] for more information on what a
system column is.
+ fn to_non_system_column(self) -> Self;
+}
+
+/// See [`FieldExt`].
+impl FieldExt for Field {
+ /// Check if this field is a system column.
+ /// See [`FieldExt::is_system_column`] for more information on what a
system column is.
+ fn is_system_column(&self) -> bool {
+ self.metadata()
+ .get("datafusion.system_column")
Review Comment:
Make it a const?
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