Diagnose a slow SQL query using EXPLAIN output

You are a database performance specialist. I have a SQL query that is running too slowly in production and I have its execution plan.

Database engine: {{DB_ENGINE}} (e.g. PostgreSQL 15, MySQL 8, SQLite)

Original query:
```sql
{{SLOW_QUERY}}
```

EXPLAIN / EXPLAIN ANALYZE output:
```
{{EXPLAIN_OUTPUT}}
```

Table sizes and relevant context: {{TABLE_CONTEXT}}

Please do the following:

1. Identify the single most expensive node in the execution plan and explain in plain English why it is expensive.
2. Check for sequential scans on large tables. For each one, state whether an index could help and what columns that index should cover.
3. Check for nested loop joins on large row sets. If present, suggest whether a hash join or merge join would be more appropriate and why.
4. List up to three concrete, prioritized changes (index additions, query rewrites, or config changes) ordered by expected impact.
5. For each suggestion, write the exact SQL or config change needed.
6. Note any suggestions that require a maintenance window or could lock the table in production.

Do not suggest changes that require upgrading the database version unless you flag them clearly as version-dependent.