Simple update query is slow

Hi,

I noticed something strange in our PG server. I have a table named
'timetable' that has only one bigint column and one row.

Once in every 5 seconds this row is updated to the current time epoch
value in milliseconds.

The update query seems to be taking considerable time (avg 50
milliseconds). When I tried generating the explain (analyze,buffers)
for the query, the planning time + execution time is always less than
0.1 millisecond. However the query time as shown when /timing of psql
is enabled shows approx 30 milliseconds (I am connecting via psql from
the localhost).


Please find the details below.

postgres=> select version();
                                                    version
----------------------------------------------------------------------------------------------------------------
 PostgreSQL 9.4.15 on x86_64-unknown-linux-gnu, compiled by gcc (GCC)
4.4.7 20120313 (Red Hat 4.4.7-18), 64-bit
(1 row)

Time: 0.572 ms





postgres=> \d+ timetable
                      Table "public.timetable"
 Column |  Type  | Modifiers | Storage | Stats target | Description
--------+--------+-----------+---------+--------------+-------------
 time   | bigint |           | plain   |              |






postgres=> table timetable ;
    time
------------
 1605988584
(1 row)

Time: 0.402 ms





postgres=> explain (analyze,buffers,verbose) update timetable set time = time+0;
                                                   QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
 Update on public.timetable  (cost=0.00..4.01 rows=1 width=14) (actual
time=0.064..0.064 rows=0 loops=1)
   Buffers: shared hit=5
   ->  Seq Scan on public.timetable  (cost=0.00..4.01 rows=1 width=14)
(actual time=0.029..0.029 rows=1 loops=1)
         Output: ("time" + 0), ctid
         Buffers: shared hit=4
 Planning time: 0.054 ms
 Execution time: 0.093 ms
(7 rows)

Time: 27.685 ms


Sometimes this shoots up to even a few hundred milliseconds.

postgres=> explain (analyze,buffers,verbose) update timetable set time = time+0;
                                                   QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
 Update on public.timetable  (cost=0.00..4.01 rows=1 width=14) (actual
time=0.048..0.048 rows=0 loops=1)
   Buffers: shared hit=5
   ->  Seq Scan on public.timetable  (cost=0.00..4.01 rows=1 width=14)
(actual time=0.027..0.028 rows=1 loops=1)
         Output: ("time" + 0), ctid
         Buffers: shared hit=4
 Planning time: 0.063 ms
 Execution time: 0.084 ms
(7 rows)

Time: 291.090 ms




I guess the problem here may somehow be linked to frequent updates to
the one row. However I want to understand what exactly is going wrong
here. Also I don't understand the discrepancy between planning +
execution time from explain analyze and the time taken by the query as
reported in pg log and in psql console.

Kindly help me on this.

Regards,
Nanda