-- =============================================
-- Education 模块 — 答案保存幂等性回滚 (PostgreSQL)
-- Ticket #7 / Migration 003
-- =============================================
--
-- WARNING: This file contains NO executable SQL.
-- Destructive rollback requires manual operator verification.
--
-- Manual rollback procedure (operator must execute):
-- 1. Verify no other tables depend on education_answer_idempotency:
--      SELECT tc.table_name, kcu.column_name, ccu.table_name AS referenced_table
--        FROM information_schema.table_constraints AS tc
--        JOIN information_schema.key_column_usage AS kcu
--          ON tc.constraint_name = kcu.constraint_name
--        JOIN information_schema.constraint_column_usage AS ccu
--          ON ccu.constraint_name = tc.constraint_name
--       WHERE tc.constraint_type = 'FOREIGN KEY'
--         AND ccu.table_name = 'education_answer_idempotency'
--         AND tc.table_catalog = current_database();
--    Result MUST be empty before proceeding.
--
-- 2. Verify the table contains only data from this migration:
--      SELECT COUNT(*) AS idempotency_count FROM education_answer_idempotency;
--    Operator must confirm this count is acceptable to destroy.
--
-- 3. Verify no application code depends on client_sequence column:
--    Search codebase for 'clientSequence' / 'client_sequence' references.
--
-- 4. After verification, execute:
--      DROP TABLE IF EXISTS education_answer_idempotency;
--      ALTER TABLE education_practice_question DROP COLUMN client_sequence;
--
-- DO NOT uncomment or execute the lines below without operator verification.
-- -- DROP TABLE IF EXISTS education_answer_idempotency;
-- -- ALTER TABLE education_practice_question DROP COLUMN client_sequence;