itsjunetime commented on code in PR #13063:
URL: https://github.com/apache/datafusion/pull/13063#discussion_r1811219153
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datafusion/core/src/datasource/schema_adapter.rs:
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@@ -99,8 +107,86 @@ pub trait SchemaMapper: Debug + Send + Sync {
) -> datafusion_common::Result<RecordBatch>;
}
-/// Implementation of [`SchemaAdapterFactory`] that maps columns by name
-/// and casts columns to the expected type.
+/// Default [`SchemaAdapterFactory`] for mapping schemas.
+///
+/// This can be used to adapt file-level record batches to a table schema and
+/// implement schema evolution.
+///
+/// Given an input file schema and a table schema, this factor can make
+/// [`SchemaMapper`]s that:
+///
+/// 1. Reorder columns
+/// 2. Cast columns to the correct type
+/// 3. Fill missing columns with nulls
+///
+/// # Illustration of Schema Mapping
+///
+/// ```text
+/// ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─
─
+/// ┌───────┐ ┌───────┐ │ ┌───────┐ ┌───────┐
┌───────┐ │
+/// ││ 1.0 │ │ "foo" │ ││ NULL │ │ "foo" │ │ "1.0" │
+/// ├───────┤ ├───────┤ │ Schema mapping ├───────┤ ├───────┤
├───────┤ │
+/// ││ 2.0 │ │ "bar" │ ││ NULL │ │ "bar" │ │ "2.0" │
+/// └───────┘ └───────┘ │────────────────▶ └───────┘ └───────┘
└───────┘ │
+/// │ │
+/// column "c" column "b"│ column "a" column "b" column
"c"│
+/// │ Float64 Utf8 │ Int32 Utf8 Utf8
+/// ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┘ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─
─ ┘
Review Comment:
As just a personal question: What do you use to create these sort of
diagrams? Do you make them by hand or is there a program that allows you to
easily draw these text-based squares and arrows and such?
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