Before a large join runs, we need to know how many rows it will produce and which keys will not match. Implement that report using the actual multiplicity on both sides of the join.
Build the implementation in SQL under /app, with /app/run.sh as its entry point. Read one JSON request from standard input and write one JSON result to standard output. Input has left and right lists of string keys. Return inner_rows counting every left/right row pair with equal keys, plus left_only and right_only sorted unique keys absent on the other side. Duplicates multiply pair counts; empty strings are ordinary keys.
Reject inputs that violate an explicit validation rule with exit status 2, no standard output, and {"error":"invalid_input"} on standard error. For valid input, exit with status 0. Keep the implementation reusable for new inputs; it must work offline and must not depend on the example data being the only data it receives.
Repair or replace the unfinished /app/run.sh and add source files under /app. Malformed JSON must use the same invalid-input response. Do not read /tests or /oracle from the application.