Compare a pyspark dataframe to another dataframe

Question

I have 2 data frames to compare both have the same number of columns and the comparison result should have the field that is mismatching and the values along with the ID.

Dataframe one

+-----+---+--------+
| name| id|    City|
+-----+---+--------+
|  Sam|  3| Toronto|
| BALU| 11|     YYY|
|CLAIR|  7|Montreal|
|HELEN| 10|  London|
|HELEN| 16|  Ottawa|
+-----+---+--------+

Dataframe two

+-------------+-----------+-------------+
|Expected_name|Expected_id|Expected_City|
+-------------+-----------+-------------+
|          SAM|          3|      Toronto|
|         BALU|         11|          YYY|
|        CLARE|          7|     Montreal|
|        HELEN|         10|        Londn|
|        HELEN|         15|       Ottawa|
+-------------+-----------+-------------+

Expected Output

+---+------------+--------------+-----+
| ID|Actual_value|Expected_value|Field|
+---+------------+--------------+-----+
|  7|       CLAIR|         CLARE| name|
|  3|         Sam|           SAM| name|
| 10|      London|         Londn| City|
+---+------------+--------------+-----+

Code

Create example data

from pyspark.sql import SQLContext
from pyspark.context import SparkContext
from pyspark.sql.functions import *
from pyspark.sql.types import StructType, StructField, IntegerType, StringType
from pyspark.sql import SparkSession

sc = SparkContext()
sql_context = SQLContext(sc)

spark = SparkSession.builder.getOrCreate()

spark.sparkContext.setLogLevel("ERROR") # log only on fails

df_Actual = sql_context.createDataFrame(
    [("Sam", 3,'Toronto'), ("BALU", 11,'YYY'), ("CLAIR", 7,'Montreal'), 
     ("HELEN", 10,'London'), ("HELEN", 16,'Ottawa')],
    ["name", "id","City"]
)

df_Expected = sql_context.createDataFrame(
     [("SAM", 3,'Toronto'), ("BALU", 11,'YYY'), ("CLARE", 7,'Montreal'), 
      ("HELEN", 10,'Londn'), ("HELEN", 15,'Ottawa')],
     ["Expected_name", "Expected_id","Expected_City"]
)

Create empty dataframe for Result

field = [
    StructField("ID",StringType(), True),
    StructField("Actual_value", StringType(), True), 
    StructField("Expected_value", StringType(), True),
    StructField("Field", StringType(), True)
]

schema = StructType(field)
Df_Result = sql_context.createDataFrame(sc.emptyRDD(), schema)

Join expected and actual on id's

df_cobined = df_Actual.join(df_Expected, (df_Actual.id == df_Expected.Expected_id))

col_names=df_Actual.schema.names

Loop through each column to find mismatches

for col_name in col_names:

    #Filter for column values not matching
    df_comp= df_cobined.filter(col(col_name)!=col("Expected_"+col_name ))\
        .select(col('id'),col(col_name),col("Expected_"+col_name ))

    #Add not matching column name
    df_comp = df_comp.withColumn("Field", lit(col_name))

    #Add to final result
    Df_Result = Df_Result.union(df_comp)
Df_Result.show()

This code works as expected. However, in the real case, I have more columns and millions of rows to compare. With this code, it takes more time to finish the comparison. Is there a better way to increase the performance and get the same result?

Answer 1

One way to avoid doing the union is the following:

• Create a list of columns to compare: to_compare
• Next select the id column and use pyspark.sql.functions.when to compare the columns. For those with a mismatch, build an array of structs with 3 fields: (Actual_value, Expected_value, Field) for each column in to_compare
• Explode the temp array column and drop the nulls
• Finally select the id and use col.* to expand the values from the struct into columns.

Code:

StructType to store the mismatched fields.

import pyspark.sql.functions as f

# these are the fields you want to compare
to_compare = [c for c in df_Actual.columns if c != "id"]

df_new = df_cobined.select(
        "id", 
        f.array([
            f.when(
                f.col(c) != f.col("Expected_"+c), 
                f.struct(
                    f.col(c).alias("Actual_value"),
                    f.col("Expected_"+c).alias("Expected_value"),
                    f.lit(c).alias("Field")
                )
            ).alias(c)
            for c in to_compare
        ]).alias("temp")
    )\
    .select("id", f.explode("temp"))\
    .dropna()\
    .select("id", "col.*")
df_new.show()
#+---+------------+--------------+-----+
#| id|Actual_value|Expected_value|Field|
#+---+------------+--------------+-----+
#|  7|       CLAIR|         CLARE| name|
#| 10|      London|         Londn| City|
#|  3|         Sam|           SAM| name|
#+---+------------+--------------+-----+

Answer 2

Join only those records where expected id equals actual and there is mismatch in any other column:

df1.join(df2, df1.id=df2.id and (df1.name != df2.name or df1.age != df2.age...))

This means you will do for loop only across mismatched rows, instead of whole dataset.