Show the optimized and non-optimized query plans — explain.polars_lazy

This function is available for LazyFrames only.

By default, explain() shows the query plan that is optimized and then run by Polars. Setting optimized = FALSE shows the query plan as-is, without any optimization done, but this is not the query performed. Note that the plans are read from bottom to top.

Usage

# S3 method for class 'polars_lazy_frame'
explain(x, optimized = TRUE, ...)

Arguments

x: A Polars LazyFrame.
optimized: Logical. If TRUE (default), show the query optimized by Polars. Otherwise, show the initial query.
...: Ignored.

Examples

query <- mtcars |>
  as_polars_lf() |>
  arrange(drat) |>
  filter(cyl == 3) |>
  select(mpg)

# unoptimized query plan:
no_opt <- explain(query, optimized = FALSE)
no_opt
#> [1] "SELECT [col(\"mpg\")]\n  FILTER [(col(\"cyl\")) == (3.0)]\n  FROM\n    SORT BY [nulls_last: [true]] [col(\"drat\")]\n      DF [\"mpg\", \"cyl\", \"disp\", \"hp\", ...]; PROJECT */11 COLUMNS"

# better printing with cat():
cat(no_opt)
#> SELECT [col("mpg")]
#>   FILTER [(col("cyl")) == (3.0)]
#>   FROM
#>     SORT BY [nulls_last: [true]] [col("drat")]
#>       DF ["mpg", "cyl", "disp", "hp", ...]; PROJECT */11 COLUMNS

# optimized query run by polars
cat(explain(query))
#> simple π 1/1 ["mpg"]
#>   SORT BY [nulls_last: [true]] [col("drat")]
#>     FILTER [(col("cyl")) == (3.0)]
#>     FROM
#>       DF ["mpg", "cyl", "disp", "hp", ...]; PROJECT["mpg", "cyl", "drat"] 3/11 COLUMNS