different results between 0.19.8 and 0.20.31

[Tmonster]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

python script for the bug

#!/usr/bin/env python3

import polars as pl
from polars import col

src_grp = "G1_1e7_1e2_5_0.csv"

with pl.StringCache():
    x = (pl.read_csv(src_grp, dtypes={"id4":pl.Int32, "id5":pl.Int32, "id6":pl.Int32, "v1":pl.Int32, "v2":pl.Int32, "v3":pl.Float64}, low_memory=True)
         .with_columns(pl.col(["id1", "id2", "id3"]).cast(pl.Categorical)))

ans = x.group_by(["id1","id2","id3","id4","id5","id6"]).agg([pl.sum("v3").alias("v3"), pl.count("v1").alias("count")])
chk = ans.lazy().select([pl.col("v3").sum(), pl.col("count").cast(pl.Int64).sum()]).collect()
print(f"chk = {chk}")

compare results with 2 different polars versions

# get the dataset 
wget https://duckdb-blobs.s3.amazonaws.com/data/G1_1e7_1e2_5_0.csv
# setup virtual environments
python3 -m venv polars-0.19.8
python3 -m venv polars-0.20.31
# run on 0.19.8
source polars-0.19.8/bin/activate
python3 -m pip install polars==0.19.8
python3 -m pip install numpy
python3 bug.py
# chk = shape: (1, 2)
# ┌──────────┬──────────┐
# │ v3       ┆ count    │
# │ ---      ┆ ---      │
# │ f64      ┆ i64      │
# ╞══════════╪══════════╡
# │ 4.7497e8 ┆ 10000000 │
# └──────────┴──────────┘
deactivate
# run on 0.20.31
source polars-0.20.31/bin/activate
python3 -m pip install polars==0.19.8
python3 -m pip install numpy
python3 bug.py
# chk answer 1 = shape: (1, 2)
#┌──────────┬─────────┐
# │ v3       ┆ count   │
# │ ---      ┆ ---     │
# │ f64      ┆ i64     │
# ╞══════════╪═════════╡
# │ 4.7497e8 ┆ 9500000 │
# └──────────┴─────────┘

Log output

No response

Issue description

The count(*) result is not consistent between versions.

Expected behavior

The output is the same

Installed versions

--------Version info---------
Polars:               0.20.31
Index type:           UInt32
Platform:             macOS-13.4.1-arm64-arm-64bit
Python:               3.12.3 (main, Apr  9 2024, 08:09:14) [Clang 15.0.0 (clang-1500.1.0.2.5)]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               <not installed>
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
nest_asyncio:         <not installed>
numpy:                1.26.4
openpyxl:             <not installed>
pandas:               <not installed>
pyarrow:              <not installed>
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           <not installed>
torch:                <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

--------Version info---------
Polars:              0.19.8
Index type:          UInt32
Platform:            macOS-13.4.1-arm64-arm-64bit
Python:              3.12.3 (main, Apr  9 2024, 08:09:14) [Clang 15.0.0 (clang-1500.1.0.2.5)]

----Optional dependencies----
adbc_driver_sqlite:  <not installed>
cloudpickle:         <not installed>
connectorx:          <not installed>
deltalake:           <not installed>
fsspec:              <not installed>
gevent:              <not installed>
matplotlib:          <not installed>
numpy:               1.26.4
openpyxl:            <not installed>
pandas:              <not installed>
pyarrow:             <not installed>
pydantic:            <not installed>
pyiceberg:           <not installed>
pyxlsb:              <not installed>
sqlalchemy:          <not installed>
xlsx2csv:            <not installed>
xlsxwriter:          <not installed>

[mcrumiller]

pl.count() has changed since 0.19.8 and 0.20.31, it now does not count null values, hence why it is returning a smalling number.

0.19

Count the number of values in this column/context.
Warning
null is deemed a value in this context.

0.20 (stable)

Return the number of non-null values in the column.
...
Calling this function without any arguments returns the number of rows in the context. This way of using the function is deprecated. Please use len() instead.

[Tmonster]

Ah ok, yea that's probably the issue. I'll close this then