Introducing lazy mode

Introduction to Polars

Liam Brannigan

Data Scientist & Polars Contributor

Eager mode

(
    pl.read_csv("vacation_rentals.csv")


)

Loads CSV into DataFrame

Lazy mode

Eager mode

(
    pl.read_csv("vacation_rentals.csv")


)

Loads CSV into DataFrame

Lazy mode

(
    pl.scan_csv("vacation_rentals.csv")


)

Starts query plan
Reads first rows of CSV

Eager mode

(
    pl.read_csv("vacation_rentals.csv")


)

| name     | type    | ... | beach |
| ---      | ---     | ... | ---   |
| str      | str     | ... | bool  |
|----------|---------| ... |-------|
| Waves    | Cottage | ... | false |
| Seashell | Cottage | ... | true  |
| ...      | ...     | ... | ...   |

Lazy mode

(
    pl.scan_csv("vacation_rentals.csv")


)

naive plan: 

Csv SCAN [vacation_rentals.csv]
PROJECT */8 COLUMNS

Eager mode

(
    pl.read_csv("vacation_rentals.csv")
    .select("name","price")

)

Lazy mode

Eager mode

(
    pl.read_csv("vacation_rentals.csv")
    .select("name","price")

)

Lazy mode

(
    pl.scan_csv("vacation_rentals.csv")
    .select("name","price")

)

Updates query plan
Optimizes query plan

Optimized query plan

print(
    pl.scan_csv("vacation_rentals.csv")
    .select("name","price")
    .explain()
)

Csv SCAN [vacation_rentals.csv]

PROJECT 2/8 COLUMNS

Optimization: projection pushdown

Executing a lazy query

(
    pl.scan_csv("vacation_rentals.csv")
    .select("name","price")
    .collect()
)

shape: (49, 2)
| name      | price |
| ---       | ---   |
| str       | i64   |
|-----------|-------|
| Waves     | 540   |
| Seashells | 540   |
| ...       | ...   |

Eager mode vs. lazy mode

(
    pl.read_csv("vacation_rentals.csv")
    .select("name","price")

)

Line-by-line

(
    pl.scan_csv("vacation_rentals.csv")
    .select("name","price")
    .collect()
)

Full query

Eager mode vs. lazy mode

A running Polar Bear and a sleeping Polars Bear

step-by-step: eager mode
debugging: eager mode
optimized performance: lazy mode

Let's practice!

Introduction to Polars