feat: Add SQL support for GREATEST and LEAST

[alexander-beedie]

(Lack of support for these functions was mentioned on the Discord).

SQL's GREATEST and LEAST functions map directly onto our max/min horizontal functions. We also already follow PostgreSQL's NULL handling (and compatible supertype) behaviour¹ here, so no special adjustments needed.

Example

import polars as pl

df = pl.DataFrame({
    "a": [-100, None, 200, 99],
    "b": [None, -0.1, 99., 9.],
    "c": [50, -10, 125, -1000],
})

with pl.SQLContext(df=df) as ctx:
    df_greatest = ctx.execute(
        """
        SELECT
          GREATEST("a", "b")    AS max_abc,
          GREATEST("a", 0, "b") AS max_abc_zero, 
          LEAST("a", "b")       AS min_abc,
          LEAST("a", 0, "b")    AS min_abc_zero,  
        FROM df
        """
    ).collect()

    # shape: (4, 4)
    # ┌─────────┬──────────────┬─────────┬──────────────┐
    # │ max_abc ┆ max_abc_zero ┆ min_abc ┆ min_abc_zero │
    # │ ---     ┆ ---          ┆ ---     ┆ ---          │
    # │ f64     ┆ f64          ┆ f64     ┆ f64          │
    # ╞═════════╪══════════════╪═════════╪══════════════╡
    # │ -100.0  ┆ 0.0          ┆ -100.0  ┆ -100.0       │
    # │ -0.1    ┆ 0.0          ┆ -0.1    ┆ -0.1         │
    # │ 200.0   ┆ 200.0        ┆ 99.0    ┆ 0.0          │
    # │ 99.0    ┆ 99.0         ┆ 9.0     ┆ 0.0          │
    # └─────────┴──────────────┴─────────┴──────────────┘

Footnotes

1. https://www.postgresql.org/docs/current/functions-conditional.html#FUNCTIONS-GREATEST-LEAST ↩

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 81.15%. Comparing base (7dfc53e) to head (6eece24).
Report is 3 commits behind head on main.

Files	Patch %	Lines
crates/polars-sql/src/functions.rs	71.42%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15511   +/-   ##
=======================================
  Coverage   81.15%   81.15%           
=======================================
  Files        1362     1362           
  Lines      174933   174989   +56     
  Branches     2531     2533    +2     
=======================================
+ Hits       141963   142010   +47     
- Misses      32487    32496    +9     
  Partials      483      483           

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[stinodego]

The SQL wizard strikes again 😄

[alexander-beedie]

The SQL wizard strikes again 😄

Lol... and I'm not done yet - TRUNCATE support incoming :))