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NEW QUESTION 49
The code block displayed below contains an error. The code block should write DataFrame transactionsDf as a parquet file to location filePath after partitioning it on column storeId. Find the error.
Code block:
transactionsDf.write.partitionOn("storeId").parquet(filePath)
Answer: C
Explanation:
Explanation
No method partitionOn() exists for the DataFrame class, partitionBy() should be used instead.
Correct! Find out more about partitionBy() in the documentation (linked below).
The operator should use the mode() option to configure the DataFrameWriter so that it replaces any existing files at location filePath.
No. There is no information about whether files should be overwritten in the question.
The partitioning column as well as the file path should be passed to the write() method of DataFrame transactionsDf directly and not as appended commands as in the code block.
Incorrect. To write a DataFrame to disk, you need to work with a DataFrameWriter object which you get access to through the DataFrame.writer property - no parentheses involved.
Column storeId should be wrapped in a col() operator.
No, this is not necessary - the problem is in the partitionOn command (see above).
The partitionOn method should be called before the write method.
Wrong. First of all partitionOn is not a valid method of DataFrame. However, even assuming partitionOn would be replaced by partitionBy (which is a valid method), this method is a method of DataFrameWriter and not of DataFrame. So, you would always have to first call DataFrame.write to get access to the DataFrameWriter object and afterwards call partitionBy.
More info: pyspark.sql.DataFrameWriter.partitionBy - PySpark 3.1.2 documentation Static notebook | Dynamic notebook: See test 3
NEW QUESTION 50
Which of the following describes a valid concern about partitioning?
Answer: A
Explanation:
Explanation
A shuffle operation returns 200 partitions if not explicitly set.
Correct. 200 is the default value for the Spark property spark.sql.shuffle.partitions. This property determines how many partitions Spark uses when shuffling data for joins or aggregations.
The coalesce() method should be used to increase the number of partitions.
Incorrect. The coalesce() method can only be used to decrease the number of partitions.
Decreasing the number of partitions reduces the overall runtime of narrow transformations if there are more executors available than partitions.
No. For narrow transformations, fewer partitions usually result in a longer overall runtime, if more executors are available than partitions.
A narrow transformation does not include a shuffle, so no data need to be exchanged between executors.
Shuffles are expensive and can be a bottleneck for executing Spark workloads.
Narrow transformations, however, are executed on a per-partition basis, blocking one executor per partition.
So, it matters how many executors are available to perform work in parallel relative to the number of partitions. If the number of executors is greater than the number of partitions, then some executors are idle while other process the partitions. On the flip side, if the number of executors is smaller than the number of partitions, the entire operation can only be finished after some executors have processed multiple partitions, one after the other. To minimize the overall runtime, one would want to have the number of partitions equal to the number of executors (but not more).
So, for the scenario at hand, increasing the number of partitions reduces the overall runtime of narrow transformations if there are more executors available than partitions.
No data is exchanged between executors when coalesce() is run.
No. While coalesce() avoids a full shuffle, it may still cause a partial shuffle, resulting in data exchange between executors.
Short partition processing times are indicative of low skew.
Incorrect. Data skew means that data is distributed unevenly over the partitions of a dataset. Low skew therefore means that data is distributed evenly.
Partition processing time, the time that executors take to process partitions, can be indicative of skew if some executors take a long time to process a partition, but others do not. However, a short processing time is not per se indicative a low skew: It may simply be short because the partition is small.
A situation indicative of low skew may be when all executors finish processing their partitions in the same timeframe. High skew may be indicated by some executors taking much longer to finish their partitions than others. But the answer does not make any comparison - so by itself it does not provide enough information to make any assessment about skew.
More info: Spark Repartition & Coalesce - Explained and Performance Tuning - Spark 3.1.2 Documentation
NEW QUESTION 51
Which of the following is the idea behind dynamic partition pruning in Spark?
Answer: B
Explanation:
Explanation
Dynamic partition pruning reoptimizes query plans based on runtime statistics collected during query execution.
No - this is what adaptive query execution does, but not dynamic partition pruning.
Dynamic partition pruning concatenates columns of similar data types to optimize join performance.
Wrong, this answer does not make sense, especially related to dynamic partition pruning.
Dynamic partition pruning reoptimizes physical plans based on data types and broadcast variables.
It is true that dynamic partition pruning works in joins using broadcast variables. This actually happens in both the logical optimization and the physical planning stage. However, data types do not play a role for the reoptimization.
Dynamic partition pruning performs wide transformations on disk instead of in memory.
This answer does not make sense. Dynamic partition pruning is meant to accelerate Spark - performing any transformation involving disk instead of memory resources would decelerate Spark and certainly achieve the opposite effect of what dynamic partition pruning is intended for.
NEW QUESTION 52
The code block displayed below contains an error. The code block should arrange the rows of DataFrame transactionsDf using information from two columns in an ordered fashion, arranging first by column value, showing smaller numbers at the top and greater numbers at the bottom, and then by column predError, for which all values should be arranged in the inverse way of the order of items in column value. Find the error.
Code block:
transactionsDf.orderBy('value', asc_nulls_first(col('predError')))
Answer: B
Explanation:
Explanation
Correct code block:
transactionsDf.orderBy('value', desc_nulls_last('predError'))
Column predError should be sorted in a descending way, putting nulls last.
Correct! By default, Spark sorts ascending, putting nulls first. So, the inverse sort of the default sort is indeed desc_nulls_last.
Instead of orderBy, sort should be used.
No. DataFrame.sort() orders data per partition, it does not guarantee a global order. This is why orderBy is the more appropriate operator here.
Column value should be wrapped by the col() operator.
Incorrect. DataFrame.sort() accepts both string and Column objects.
Column predError should be sorted by desc_nulls_first() instead.
Wrong. Since Spark's default sort order matches asc_nulls_first(), nulls would have to come last when inverted.
Two orderBy statements with calls to the individual columns should be chained, instead of having both columns in one orderBy statement.
No, this would just sort the DataFrame by the very last column, but would not take information from both columns into account, as noted in the question.
More info: pyspark.sql.DataFrame.orderBy - PySpark 3.1.2 documentation, pyspark.sql.functions.desc_nulls_last - PySpark 3.1.2 documentation, sort() vs orderBy() in Spark | Towards Data Science Static notebook | Dynamic notebook: See test 3
NEW QUESTION 53
Which of the following code blocks returns a DataFrame with an added column to DataFrame transactionsDf that shows the unix epoch timestamps in column transactionDate as strings in the format month/day/year in column transactionDateFormatted?
Excerpt of DataFrame transactionsDf:
Answer: E
Explanation:
Explanation
transactionsDf.withColumn("transactionDateFormatted", from_unixtime("transactionDate", format="MM/dd/yyyy")) Correct. This code block adds a new column with the name transactionDateFormatted to DataFrame transactionsDf, using Spark's from_unixtime method to transform values in column transactionDate into strings, following the format requested in the question.
transactionsDf.withColumn("transactionDateFormatted", from_unixtime("transactionDate", format="dd/MM/yyyy")) No. Although almost correct, this uses the wrong format for the timestamp to date conversion: day/month/year instead of month/day/year.
transactionsDf.withColumnRenamed("transactionDate", "transactionDateFormatted", from_unixtime("transactionDateFormatted", format="MM/dd/yyyy")) Incorrect. This answer uses wrong syntax. The command DataFrame.withColumnRenamed() is for renaming an existing column only has two string parameters, specifying the old and the new name of the column.
transactionsDf.apply(from_unixtime(format="MM/dd/yyyy")).asColumn("transactionDateFormatted") Wrong. Although this answer looks very tempting, it is actually incorrect Spark syntax. In Spark, there is no method DataFrame.apply(). Spark has an apply() method that can be used on grouped data - but this is irrelevant for this question, since we do not deal with grouped data here.
transactionsDf.withColumn("transactionDateFormatted", from_unixtime("transactionDate")) No. Although this is valid Spark syntax, the strings in column transactionDateFormatted would look like this:
2020-04-26 15:35:32, the default format specified in Spark for from_unixtime and not what is asked for in the question.
More info: pyspark.sql.functions.from_unixtime - PySpark 3.1.1 documentation and pyspark.sql.DataFrame.withColumnRenamed - PySpark 3.1.1 documentation Static notebook | Dynamic notebook: See test 1
NEW QUESTION 54
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