Example Usage¶

In this example some simple processing is performed on the example.csv file.

Even more code examples attached to the project.

Add published DataFusion dependency¶

Find latest available Datafusion version on DataFusion’s crates.io page. Add the dependency to your Cargo.toml file:

datafusion = "latest_version"
tokio = { version = "1.0", features = ["rt-multi-thread"] }

Run a SQL query against data stored in a CSV¶

use datafusion::prelude::*;

#[tokio::main]
async fn main() -> datafusion::error::Result<()> {
  // register the table
  let ctx = SessionContext::new();
  ctx.register_csv("example", "tests/data/example.csv", CsvReadOptions::new()).await?;

  // create a plan to run a SQL query
  let df = ctx.sql("SELECT a, MIN(b) FROM example WHERE a <= b GROUP BY a LIMIT 100").await?;

  // execute and print results
  df.show().await?;
  Ok(())
}

See the SQL API section of the library guide for more information on the SQL API.

Use the DataFrame API to process data stored in a CSV¶

use datafusion::prelude::*;
use datafusion::functions_aggregate::expr_fn::min;

#[tokio::main]
async fn main() -> datafusion::error::Result<()> {
  // create the dataframe
  let ctx = SessionContext::new();
  let df = ctx.read_csv("tests/data/example.csv", CsvReadOptions::new()).await?;

  let df = df.filter(col("a").lt_eq(col("b")))?
           .aggregate(vec![col("a")], vec![min(col("b"))])?
           .limit(0, Some(100))?;

  // execute and print results
  df.show().await?;
  Ok(())
}

Output from both examples¶

+---+--------+
| a | MIN(b) |
+---+--------+
| 1 | 2      |
+---+--------+

Arrow Versions¶

Many of DataFusion’s public APIs use types from the arrow and parquet crates, so if you use arrow in your project, the arrow version must match that used by DataFusion. You can check the required version on DataFusion’s crates.io page.

The easiest way to ensure the versions match is to use the arrow exported by DataFusion, for example:

use datafusion::arrow::datatypes::Schema;

For example, [DataFusion 25.0.0 dependencies] require arrow 39.0.0. If instead you used arrow 40.0.0 in your project you may see errors such as:

mismatched types [E0308] expected `Schema`, found `arrow_schema::Schema` Note: `arrow_schema::Schema` and `Schema` have similar names, but are actually distinct types Note: `arrow_schema::Schema` is defined in crate `arrow_schema` Note: `Schema` is defined in crate `arrow_schema` Note: perhaps two different versions of crate `arrow_schema` are being used? Note: associated function defined here

Or calling downcast_ref on an ArrayRef may return None unexpectedly.

Identifiers and Capitalization¶

Please be aware that all identifiers are effectively made lower-case in SQL, so if your csv file has capital letters (ex: Name) you must put your column name in double quotes or the examples won’t work.

To illustrate this behavior, consider the capitalized_example.csv file:

Run a SQL query against data stored in a CSV:¶

use datafusion::prelude::*;

#[tokio::main]
async fn main() -> datafusion::error::Result<()> {
  // register the table
  let ctx = SessionContext::new();
  ctx.register_csv("example", "tests/data/capitalized_example.csv", CsvReadOptions::new()).await?;

  // create a plan to run a SQL query
  let df = ctx.sql("SELECT \"A\", MIN(b) FROM example WHERE \"A\" <= c GROUP BY \"A\" LIMIT 100").await?;

  // execute and print results
  df.show().await?;
  Ok(())
}

Use the DataFrame API to process data stored in a CSV:¶

use datafusion::prelude::*;
use datafusion::functions_aggregate::expr_fn::min;

#[tokio::main]
async fn main() -> datafusion::error::Result<()> {
  // create the dataframe
  let ctx = SessionContext::new();
  let df = ctx.read_csv("tests/data/capitalized_example.csv", CsvReadOptions::new()).await?;

  let df = df
      // col will parse the input string, hence requiring double quotes to maintain the capitalization
      .filter(col("\"A\"").lt_eq(col("c")))?
      // alternatively use ident to pass in an unqualified column name directly without parsing
      .aggregate(vec![ident("A")], vec![min(col("b"))])?
      .limit(0, Some(100))?;

  // execute and print results
  df.show().await?;
  Ok(())
}

Output from both examples¶

+---+--------+
| A | MIN(b) |
+---+--------+
| 2 | 1      |
| 1 | 2      |
+---+--------+

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