Creating a materialized view (Option B) is the most effective way to optimize the performance of querying a directory table. Materialized views pre-compute and store the results of a query, allowing for faster retrieval of data, especially when the underlying table (the directory table in this case) is large. Increasing warehouse size (Option A) can help to a certain extent, but a materialized view provides a more significant performance boost. Refreshing the directory table (Option C) keeps the metadata up- to-date but doesn't directly improve query performance.
Creating an index (Option D) is not possible on directory tables. While reducing the number of files (Option E) might help slightly, it's often impractical and doesn't address the underlying issue of querying a large dataset. It's best to handle millions of files with proper indexing and data partitioning.