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Streaming API

One additional benefit of the lazy API is that it allows queries to be executed in a streaming manner. Instead of processing the data all-at-once Polars can execute the query in batches allowing you to process datasets that are larger-than-memory.

To tell Polars we want to execute a query in streaming mode we pass the streaming=True argument to collect

collect 

q = (
    pl.scan_csv("docs/data/iris.csv")
    .filter(pl.col("sepal_length") > 5)
    .group_by("species")
    .agg(pl.col("sepal_width").mean())
)

df = q.collect(streaming=True)

collect ยท Available on feature streaming 

let q = LazyCsvReader::new("docs/data/iris.csv")
    .has_header(true)
    .finish()?
    .filter(col("sepal_length").gt(lit(5)))
    .group_by(vec![col("species")])
    .agg([col("sepal_width").mean()]);

let df = q.with_streaming(true).collect()?;
println!("{}", df);

When is streaming available?

Streaming is still in development. We can ask Polars to execute any lazy query in streaming mode. However, not all lazy operations support streaming. If there is an operation for which streaming is not supported Polars will run the query in non-streaming mode.

Streaming is supported for many operations including:

  • filter,slice,head,tail
  • with_columns,select
  • group_by
  • join
  • sort
  • explode,melt
  • scan_csv,scan_parquet,scan_ipc