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Support marking columns as system columns via metadata
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| // Licensed to the Apache Software Foundation (ASF) under one | ||
| // or more contributor license agreements. See the NOTICE file | ||
| // distributed with this work for additional information | ||
| // regarding copyright ownership. The ASF licenses this file | ||
| // to you under the Apache License, Version 2.0 (the | ||
| // "License"); you may not use this file except in compliance | ||
| // with the License. You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, | ||
| // software distributed under the License is distributed on an | ||
| // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY | ||
| // KIND, either express or implied. See the License for the | ||
| // specific language governing permissions and limitations | ||
| // under the License. | ||
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| use std::collections::HashMap; | ||
| use std::sync::Arc; | ||
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| use arrow::array::record_batch; | ||
| use datafusion::arrow::datatypes::{DataType, Field, Schema}; | ||
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| use datafusion::{assert_batches_eq, prelude::*}; | ||
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| /// This example shows how to mark fields as 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 | | ||
| /// +-----+---+ | ||
| /// ``` | ||
| #[tokio::main] | ||
| async fn main() { | ||
| let batch = record_batch!( | ||
| ("a", Int32, [1, 2, 3]), | ||
| ("b", Utf8, ["foo", "bar", "baz"]), | ||
| ("_row_num", UInt32, [1, 2, 3]) | ||
| ).unwrap(); | ||
| let batch = batch.with_schema( | ||
| Arc::new( | ||
| Schema::new(vec![ | ||
| Field::new("a", DataType::Int32, true), | ||
| Field::new("b", DataType::Utf8, true), | ||
| Field::new("_row_num", DataType::UInt32, true).with_metadata(HashMap::from_iter([ | ||
| ("datafusion.system_column".to_string(), "true".to_string()), | ||
| ])), | ||
| ]) | ||
| ) | ||
| ).unwrap(); | ||
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| let ctx = SessionContext::new(); | ||
| let _ = ctx.register_batch("t", batch); | ||
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| let res = ctx.sql("SELECT a, b FROM t").await.unwrap().collect().await.unwrap(); | ||
| #[rustfmt::skip] | ||
| let expected: Vec<&str> = vec![ | ||
| "+---+-----+", | ||
| "| a | b |", | ||
| "+---+-----+", | ||
| "| 1 | foo |", | ||
| "| 2 | bar |", | ||
| "| 3 | baz |", | ||
| "+---+-----+", | ||
| ]; | ||
| assert_batches_eq!(expected, &res); | ||
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| let res = ctx.sql("SELECT _row_num FROM t").await.unwrap().collect().await.unwrap(); | ||
| #[rustfmt::skip] | ||
| let expected: Vec<&str> = vec![ | ||
| "+----------+", | ||
| "| _row_num |", | ||
| "+----------+", | ||
| "| 1 |", | ||
| "| 2 |", | ||
| "| 3 |", | ||
| "+----------+", | ||
| ]; | ||
| assert_batches_eq!(expected, &res); | ||
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| let res = ctx.sql("SELECT * FROM t").await.unwrap().collect().await.unwrap(); | ||
| // does not include _row_num | ||
| #[rustfmt::skip] | ||
| let expected: Vec<&str> = vec![ | ||
| "+---+-----+", | ||
| "| a | b |", | ||
| "+---+-----+", | ||
| "| 1 | foo |", | ||
| "| 2 | bar |", | ||
| "| 3 | baz |", | ||
| "+---+-----+", | ||
| ]; | ||
| assert_batches_eq!(expected, &res); | ||
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| let res = ctx.sql("SELECT *, _row_num FROM t").await.unwrap().collect().await.unwrap(); | ||
| #[rustfmt::skip] | ||
| let expected: Vec<&str> = vec![ | ||
| "+---+-----+----------+", | ||
| "| a | b | _row_num |", | ||
| "+---+-----+----------+", | ||
| "| 1 | foo | 1 |", | ||
| "| 2 | bar | 2 |", | ||
| "| 3 | baz | 3 |", | ||
| "+---+-----+----------+", | ||
| ]; | ||
| assert_batches_eq!(expected, &res); | ||
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| let res = ctx.sql("SELECT t._row_num FROM t").await.unwrap().collect().await.unwrap(); | ||
| #[rustfmt::skip] | ||
| let expected: Vec<&str> = vec![ | ||
| "+----------+", | ||
| "| _row_num |", | ||
| "+----------+", | ||
| "| 1 |", | ||
| "| 2 |", | ||
| "| 3 |", | ||
| "+----------+", | ||
| ]; | ||
| assert_batches_eq!(expected, &res); | ||
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| let res = ctx.sql("SELECT t.* FROM t").await.unwrap().collect().await.unwrap(); | ||
| // does not include _row_num | ||
| #[rustfmt::skip] | ||
| let expected: Vec<&str> = vec![ | ||
| "+---+-----+", | ||
| "| a | b |", | ||
| "+---+-----+", | ||
| "| 1 | foo |", | ||
| "| 2 | bar |", | ||
| "| 3 | baz |", | ||
| "+---+-----+", | ||
| ]; | ||
| assert_batches_eq!(expected, &res); | ||
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| let res = ctx.sql("SELECT t.*, _row_num FROM t").await.unwrap().collect().await.unwrap(); | ||
| #[rustfmt::skip] | ||
| let expected: Vec<&str> = vec![ | ||
| "+---+-----+----------+", | ||
| "| a | b | _row_num |", | ||
| "+---+-----+----------+", | ||
| "| 1 | foo | 1 |", | ||
| "| 2 | bar | 2 |", | ||
| "| 3 | baz | 3 |", | ||
| "+---+-----+----------+", | ||
| ]; | ||
| assert_batches_eq!(expected, &res); | ||
| } |
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