Spark ignores data types when filtering on equality

March 19th, 2020 by Ankur Gupta

This may be surprising to regular pandas users and may lead to unexpected or silent errors.

The problem

Let’s construct an example dataframe to demonstrate the problem. The following dataframe has two columns – column x has type integer, column y has type string. Column x has an element 1 (integer) and column y has an element "1" (string).

df = spark.createDataFrame([(1, 'a'), (2, 'b'), (3, '1')], 'x INT, y STRING')
df.dtypes
# [('x', 'int'), ('y', 'string')]
df.show()
# +---+---+
# |  x|  y|
# +---+---+
# |  1|  a|
# |  2|  b|
# |  3|  1|
# +---+---+

Let’s say we want to count how many 1 (integer) values are in each column. We should get back these results – column x should have one instance of 1 (integer) and column y should have zero instances of 1 (integer). But only one of these happen.

df[df['x'].isin(1)].count()  # 1 (correct)
df[df['y'].isin(1)].count()  # 1 (incorrect)

We may suspect that the pyspark.sql.Column.isin method method has a bug. Let’s check another way without using .isin method.

df[df['x'] == 1].count()  # 1 (correct)
df[df['y'] == 1].count()  # 1 (incorrect)

We still get the same partially incorrect results. Let’s check something else – filter for "1" (string) instead.

df[df['x'].isin("1")].count()  # 1 (incorrect)
df[df['y'].isin("1")].count()  # 1 (correct)

df[df['x'] == "1"].count()  # 1 (incorrect)
df[df['y'] == "1"].count()  # 1 (correct)

It appears that (1) .isin is not the reason and (2) spark does not respect data types when filtering. Python is a dynamically typed language (though it now supports optional type checking). Perhaps that is causing this. Let’s just use SQL (via python) instead and verify.

df.createOrReplaceTempView('dummy')
spark.sql('SELECT * FROM dummy WHERE x = 1').count()  # 1 (correct)
spark.sql('SELECT * FROM dummy WHERE y = 1').count()  # 1 (incorrect)
spark.sql('SELECT * FROM dummy WHERE x = "1"').count()  # 1 (incorrect)
spark.sql('SELECT * FROM dummy WHERE y = "1"').count()  # 1 (correct)

We get the same results back – data types are not respected.

Let’s just use Scala

Let’s investigate if this happens in a statically typed language like Scala.

import spark.implicits._

val df = Seq(
    (1, "a"),
    (2, "b"),
    (3, "1")
).toDF("x", "y")

df.dtypes
// Array[(String, String)] = Array((x,IntegerType), (y,StringType))

df.filter($"x" === 1).count()    // 1 (correct)
df.filter($"y" === 1).count()    // 1 (incorrect)
df.filter($"x" === "1").count()  // 1 (incorrect)
df.filter($"y" === 1).count()    // 1 (correct)

Scala has the same problem!

Plain-old SQL has the same behavior

Turns out we see the same behavior in at least some SQL systems. Let’s use SQL (using sqlite3) without using spark at all.

import sqlite3
conn = sqlite3.connect(':memory:')
c = conn.cursor()
c.execute('CREATE TABLE dummy (x integer, y string)')
c.execute('INSERT INTO dummy VALUES (1, "a")')
c.execute('INSERT INTO dummy VALUES (2, "b")')
c.execute('INSERT INTO dummy VALUES (3, "1")')
conn.commit()
list(c.execute('SELECT * FROM dummy'))
list(c.execute('SELECT * FROM dummy WHERE x=1'))   # [(1, 'a')]  (correct)
list(c.execute('SELECT * FROM dummy WHERE y=1'))   # [(3, 1)]    (incorrect)
list(c.execute('SELECT * FROM dummy WHERE x="1"')) # [(1, 'a')]  (incorrect)
list(c.execute('SELECT * FROM dummy WHERE y="1"')) # [(3, 1)]    (correct)
conn.close()

Turns out SQL does not respect data types either!

It gets worse! Even `pandas` has this bug.

Turns out that even pandas has this problem. For pyspark and SQL, this problem appears to be a consistent design issue but for pandas this problem appears to be bug instead of a design choice. I used pandas version 0.25.3 for this experiment.

pdf['x'].isin([1]).sum()  # 1 (correct)
pdf['y'].isin([1]).sum()  # 0 (correct)
pdf['x'].isin(["1"]).sum()  # 1 (incorrect)
pdf['y'].isin(["1"]).sum()  # 1 (correct)

Note how only the third line in the above snippet returns incorrect results. This indicates that pandas ignores the data type sometimes but not always. Further, in the case of pandas the problem is only in the .isin method and not in general (like in SQL and pyspark), as shown by the following example that does not use .isin.

# Results are correct when we don't use pd.Series.isin() method
pdf['x'].apply(lambda x: x == 1).sum()   # 1 (correct)
pdf['y'].apply(lambda y: y == 1).sum()   # 0 (correct)
pdf['x'].apply(lambda x: x == "1").sum() # 0 (correct)
pdf['y'].apply(lambda y: y == "1").sum() # 1 (correct)

(pdf['x'] == 1).sum()    # 1 (correct)
(pdf['y'] == 1).sum()    # 0 (correct)
(pdf['x'] == "1").sum()  # 0 (correct)
(pdf['y'] == "1").sum()  # 1 (correct)

Looking even deeper into the pandas.Series.isin method, we see that it relies upon the pandas.core.algorithms.isin function. Lines 452-453 (in pandas version 0.25.3) contain the following code (comments are mine):

# Lines 452-453 in pandas 0.25.3
comps, dtype, _ = _ensure_data(comps)              # comps = elements of the column
values, _, _ = _ensure_data(values, dtype=dtype)   # values = list of values passed to `.isin`

As you can see, values get type casted into the dtype of the column. We can verify this by actually running the code ourselves.

from pandas.core.algorithms import _ensure_data
comps = pdf['x']
values = ["1"]  # str
comps, dtype, _ = _ensure_data(comps)
print(dtype)  # int64
values, _, _ = _ensure_data(values, dtype=dtype)
print(values)  # [1]
print(values.dtype)  # int64

The reason why this does not affect column y in pdf above is because _ensure_data returns dtype=object for column y. Searching pandas GitHub issues, we see that this bug has been brought up before but was somehow ignored.

Takeaway

Spark and SQL ignore data types when filtering on equality. This seems to be a design issue and is consistent throughout (or at least as far as I can see). Pandas, on the other hand, exhibits this problem inconsistently which may lead to complacency.