Hi all,

My colleague noticed that the output of EXPLAIN ANALYZE doesn't work
fine for parallel seq scan.

postgres(1)=# explain analyze verbose select count(*) from pgbench_accounts ;
                                                    QUERY PLAN
 Finalize Aggregate  (cost=217018.55..217018.56 rows=1 width=8)
(actual time=2640.015..2640.015 rows=1 loops=1)
   Output: count(*)
   ->  Gather  (cost=217018.33..217018.54 rows=2 width=8) (actual
time=2639.064..2640.002 rows=3 loops=1)
         Output: (PARTIAL count(*))
         Workers Planned: 2
         Workers Launched: 2
         ->  Partial Aggregate  (cost=216018.33..216018.34 rows=1
width=8) (actual time=2632.714..2632.715 rows=1 loops=3)
               Output: PARTIAL count(*)
               Worker 0: actual time=2632.583..2632.584 rows=1 loops=1
               Worker 1: actual time=2627.517..2627.517 rows=1 loops=1
               ->  Parallel Seq Scan on public.pgbench_accounts
(cost=0.00..205601.67 rows=4166667 width=0) (actual
time=0.042..1685.542 rows=3333333 loops=3)
                     Worker 0: actual time=0.033..1657.486 rows=3457968 loops=1
                     Worker 1: actual time=0.039..1702.979 rows=3741069 loops=1
 Planning time: 1.026 ms
 Execution time: 2640.225 ms
(15 rows)

For example, the above result shows,
Parallel Seq Scan : actual rows = 3333333
worker 0               : actual rows = 3457968
worker 1               : actual rows = 3741069
Summation of these is 10532370, but actual total rows is 10000000.
I think that Parallel Seq Scan should show actual rows =
10000000(total rows) or actual rows = 2800963(rows collected by
itself). (10000000 maybe better)

After spent time to investigate this behaviour, ISTM that the problem
is nloops of Parallel Seq Scan.
Parallel Seq Scan is done only once, but nloops is incremented to 3.
So its "actual rows" is calculated 3333333(10000000 / 3) at explain.c:L1223.
Is it a bug?


Masahiko Sawada

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