Open temp files in PostgreSQL

Do not manually delete temp file. As a first step i would check what operations are using temp files. You have some options to enable logs on queries writing to temp files:

1. Enable log_tmp_files

or

2. Use pgbadger

3. Query table Pg_stat_statements, as follows:

   SELECT interval ‘1 millisecond’ * total_time AS total_exec_time,
   total_time / calls AS avg_exec_time_ms,
   temp_blks_written,
   query AS query
   FROM pg_stat_statements
   WHERE temp_blks_written > 0
   ORDER BY temp_blks_written DESC
   LIMIT 20;

4. Query pg_stat_database to give db level stats, as follows:

   select datname, temp_files , pg_size_pretty(temp_bytes) as temp_file_size FROM pg_stat_database order by temp_bytes desc;

5. Run EXPLAIN ANALYZE and compare work_mem size with values printed by explain analyze.

As a general recommendation set temp_file_limit to a value different than -1 (default).
Be sure to use correct indices to tune queries.
Tune work_mem (carefully).
Set statement_timeout to an optimal value.

Please note:
Temp files are created when memory is not sufficient.