Projection pushdown

The Projection pushdown page is under construction.

Let's expand our query from the previous section by joining the result of the FILTER operation with the runescape data to find popular Reddit usernames that have a username starting with an "a" that also played Runescape. That must be something we are all interested in!

The query would look like this:

import polars as pl

from ..paths import DATA_DIR

reddit = (
    .filter(pl.col("comment_karma") > 0)
    .filter(pl.col("link_karma") > 0)

runescape = pl.scan_csv("data/runescape.csv", has_headers=False).select(pl.col("column_1").alias("name"))

dataset = reddit.join(runescape, on="name", how="inner").select(["name", "comment_karma", "link_karma"])

df1 = dataset.fetch(int(1e7))
df2 = dataset.fetch(int(1e7), predicate_pushdown=True, projection_pushdown=True)

And yields the following DataFrame.

shape: (0, 3)
│ name ┆ comment_karma ┆ link_karma │
│ ---  ┆ ---           ┆ ---        │
│ str  ┆ i64           ┆ i64        │

Break it down

Again, let's take a look the query plan.


Now were focussed on the projection's indicated with π. The first node shows π 3/6, indicating that we select 3 out of 6 columns in the DataFrame. If we look the csv scans we see a wildcard π */6 and π */1 meaning that we select all of 6 columns of the reddit dataset and the one and only column from the runescape dataset respectively.

This query is not very optimal. We select all columns from both datasets and only show 3/6 after join. That means that there were some columns computed during the join operation that could have been ignored. There were also columns parsed during csv scanning only to be dropped at the end. When we are dealing with DataFrames with a large number of columns the redundant work that is done can be huge.

Optimized query

Let's see how Polars optimizes this query.


The projections are pushed down the join operation all the way to the csv scans. This means that both the scanning and join operation have become cheaper due to the query optimization.


Let's time the result before and after optimization.

Without optimization, predicate_pushdown=False and projection_pushdown=False.

real	0m3,273s
user	0m9,284s
sys	0m1,081s

With optimization, predicate_pushdown and projection_pushdown flags both to True.

real	0m1,732s
user	0m7,581s
sys	0m0,783s

We can see that we almost reduced query time by half on this simple query. With real business data often comprising of many columns, filtering missing data, doing complex groupby operations, and using joins we expect this difference between unoptimized queries and optimized queries to only grow.