datafusion-examples/examples/dataframe.rs

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//   http://www.apache.org/licenses/LICENSE-2.0
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use datafusion::arrow::datatypes::{DataType, Field, Schema};
use datafusion::error::Result;
use datafusion::prelude::*;
use std::fs;

/// This example demonstrates executing a simple query against an Arrow data source (Parquet) and
/// fetching results, using the DataFrame trait
#[tokio::main]
async fn main() -> Result<()> {
    // create local execution context
    let ctx = SessionContext::new();

    let testdata = datafusion::test_util::parquet_test_data();

    let filename = &format!("{testdata}/alltypes_plain.parquet");

    // define the query using the DataFrame trait
    let df = ctx
        .read_parquet(filename, ParquetReadOptions::default())
        .await?
        .select_columns(&["id", "bool_col", "timestamp_col"])?
        .filter(col("id").gt(lit(1)))?;

    // print the results
    df.show().await?;

    // Reading CSV file with inferred schema example
    let csv_df = example_read_csv_file_with_inferred_schema().await;
    csv_df.show().await?;

    // Reading CSV file with defined schema
    let csv_df = example_read_csv_file_with_schema().await;
    csv_df.show().await?;

    Ok(())
}

// Function to create an test CSV file
fn create_csv_file(path: String) {
    // Create the data to put into the csv file with headers
    let content = r#"id,time,vote,unixtime,rating
a1,"10 6, 2013",3,1381017600,5.0
a2,"08 9, 2013",2,1376006400,4.5"#;
    // write the data
    fs::write(path, content).expect("Problem with writing file!");
}

// Example to read data from a csv file with inferred schema
async fn example_read_csv_file_with_inferred_schema() -> DataFrame {
    let path = "example.csv";
    // Create a csv file using the predefined function
    create_csv_file(path.to_string());
    // Create a session context
    let ctx = SessionContext::new();
    // Register a lazy DataFrame using the context
    ctx.read_csv(path, CsvReadOptions::default()).await.unwrap()
}

// Example to read csv file with a defined schema for the csv file
async fn example_read_csv_file_with_schema() -> DataFrame {
    let path = "example.csv";
    // Create a csv file using the predefined function
    create_csv_file(path.to_string());
    // Create a session context
    let ctx = SessionContext::new();
    // Define the schema
    let schema = Schema::new(vec![
        Field::new("id", DataType::Utf8, false),
        Field::new("time", DataType::Utf8, false),
        Field::new("vote", DataType::Int32, true),
        Field::new("unixtime", DataType::Int64, false),
        Field::new("rating", DataType::Float32, true),
    ]);
    // Create a csv option provider with the desired schema
    let csv_read_option = CsvReadOptions {
        // Update the option provider with the defined schema
        schema: Some(&schema),
        ..Default::default()
    };
    // Register a lazy DataFrame by using the context and option provider
    ctx.read_csv(path, csv_read_option).await.unwrap()
}