"Partition by" as the name specifies, splits the table into multiple partitions. You can specify only one column as part of the PARTITION by clause. Its recommended to use the columns that are most likely to be used as part of the filter conditions as the partition key. Partition key helps with data pruning, reducing the amount of data scanned by the queries thereby reducing the overall cost. As of this writing, Bigquery only supports DATE, Timestamp and Numeric columns to be used as a Partition by column
