PDE Question 53

Community Discussion

Written by other learners. Comments can disagree with the marked answer — check the official documentation when in doubt.

• Chose D1

  D - stands for Data Skew The Read section of the query plan shows a heavy concentration of processing in one area (as indicated by the pink bar being much longer than the purple bar). This typically indicates data skew, where the majority of the data is processed by a small subset of nodes.

• Chose D3

  The most likely cause of the delay for this query is option D. Most rows in the [myproject:mydataset.mytable] table have the same value in the country column, causing data skew. Group by queries in BigQuery can run slowly when there is significant data skew on the grouped columns. Since the query is grouping by country, if most rows have the same country value, all that data will need to be shuffled to a single reducer to perform the aggregation. This can cause a data skew slowdown. Options A and B might cause general slowness but are unlikely to affect this specific grouping query. Option C could also cause some slowness but not to the degree that heavy data skew on the grouped column could. So D is the most likely root cause. Optimizing the data distribution to reduce skew on the grouped column would likely speed up this query.

• 1

  Data skew is when one or some partitions have significantly more data compared to other partitions. Data-skew is usually the result of operations that require re-partitioning the data, mostly join and grouping ( GroupBy ) operations. So D.