You are a Splunk consultant for a financial services firm. They have a large lookup table containing customer account numbers and risk scores. This lookup is used in a critical compliance search that runs every hour. The search is failing with a memory error 'The search coordinator stopped the search due to memory usage'. You have already tried increasing the memory limit for the search via limits.conf, but the error persists. The lookup file is a CSV file of 2GB, with approximately 20 million rows. The search is: index=compliance sourcetype=transactions | lookup risk_scores.csv account_id OUTPUT risk_score | stats avg(risk_score) by transaction_type. The search runs on a single search head with 16GB RAM. The lookup is defined as static. What is the most effective optimization to resolve the memory error?