What is 'Log Memory' in Query Store 2017
If we look at the documentation for the underlying object, sys.query_store_runtime_stats, we'll see it has the following descriptions:


avg_log_bytes_used - Average number of bytes in the database log used by the query plan, within the aggregation interval.Note: Azure SQL Data Warehouse will always return zero (0).
last_log_bytes_used - Number of bytes in the database log used by the last execution of the query plan, within the aggregation interval. Note: Azure SQL Data Warehouse will always return zero (0).
min_log_bytes_used - Minimum number of bytes in the database log used by the query plan, within the aggregation interval.  Note: Azure SQL Data Warehouse will always return zero (0).
max_log_bytes_used - Maximum number of bytes in the database log used by the query plan, within the aggregation interval.Note: Azure SQL Data Warehouse will always return zero (0).
stdev_log_bytes_used - Standard deviation of the number of bytes in the database log used by a query plan, within the aggregation interval.Note: Azure SQL Data Warehouse will always return zero (0).