Re: plan_rows confusion with parallel queries - Mailing list pgsql-hackers

On 11/02/2016 11:56 PM, Tomas Vondra wrote:
> On 11/02/2016 09:00 PM, Tom Lane wrote:
>> Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>>> while eye-balling some explain plans for parallel queries, I got a bit
>>> confused by the row count estimates. I wonder whether I'm alone.
>>
>> I got confused by that a minute ago, so no you're not alone.  The problem
>> is even worse in join cases.  For example:
>>
>>  Gather  (cost=34332.00..53265.35 rows=100 width=8)
>>    Workers Planned: 2
>>    ->  Hash Join  (cost=33332.00..52255.35 rows=100 width=8)
>>          Hash Cond: ((pp.f1 = cc.f1) AND (pp.f2 = cc.f2))
>>          ->  Append  (cost=0.00..8614.96 rows=417996 width=8)
>>                ->  Parallel Seq Scan on pp  (cost=0.00..8591.67
>> rows=416667 widt
>> h=8)
>>                ->  Parallel Seq Scan on pp1  (cost=0.00..23.29
>> rows=1329 width=8
>> )
>>          ->  Hash  (cost=14425.00..14425.00 rows=1000000 width=8)
>>                ->  Seq Scan on cc  (cost=0.00..14425.00 rows=1000000
>> width=8)
>>
>> There are actually 1000000 rows in pp, and none in pp1.  I'm not bothered
>> particularly by the nonzero estimate for pp1, because I know where that
>> came from, but I'm not very happy that nowhere here does it look like
>> it's estimating a million-plus rows going into the join.
>>

Although - it is estimating 1M rows, but only "per worker" estimates are 
shown, and because there are 2 workers planned it says 1M/2.4 which is 
the 416k. I agree it's a bit unclear, but at least it's consistent with 
how we treat loops (i.e. that the numbers are per loop).

But there's more fun with joins - consider for example this simple join:
                               QUERY PLAN
------------------------------------------------------------------------------ Gather  (cost=19515.96..43404.82
rows=96957width=12)         (actual time=295.167..746.312 rows=99999 loops=1)   Workers Planned: 2   Workers Launched:
2  ->  Hash Join  (cost=18515.96..32709.12 rows=96957 width=12)                  (actual time=249.281..670.309
rows=33333loops=3)         Hash Cond: (t2.a = t1.a)         ->  Parallel Seq Scan on t2             (cost=0.00..8591.67
rows=416667width=8)             (actual time=0.100..184.315 rows=333333 loops=3)         ->  Hash
(cost=16925.00..16925.00rows=96957 width=8)                   (actual time=246.760..246.760 rows=99999 loops=3)
     Buckets: 131072  Batches: 2  Memory Usage: 2976kB               ->  Seq Scan on t1
(cost=0.00..16925.00rows=96957 width=8)                   (actual time=0.065..178.385 rows=99999 loops=3)
     Filter: (b < 100000)                     Rows Removed by Filter: 900001 Planning time: 0.763 ms Execution time:
793.653ms
 
(13 rows)

Suddenly we don't show per-worker estimates for the hash join - both the 
Hash Join and the Gather have exactly the same cardinality estimate.

Now, let's try forcing Nested Loops and see what happens:
                                QUERY PLAN
----------------------------------------------------------------------------- Gather  (cost=1000.42..50559.65
rows=96957width=12)         (actual time=0.610..203.694 rows=99999 loops=1)   Workers Planned: 2   Workers Launched: 2
->  Nested Loop  (cost=0.42..39863.95 rows=96957 width=12)                    (actual time=0.222..182.755 rows=33333
loops=3)        ->  Parallel Seq Scan on t1                    (cost=0.00..9633.33 rows=40399 width=8)
 (actual time=0.030..40.358 rows=33333 loops=3)               Filter: (b < 100000)               Rows Removed by
Filter:300000         ->  Index Scan using t2_a_idx on t2              (cost=0.42..0.74 rows=1 width=8)
(actualtime=0.002..0.002 rows=1 loops=99999)               Index Cond: (a = t1.a) Planning time: 0.732 ms Execution
time:250.707 ms
 
(11 rows)

So, different join method but same result - 2 workers, loops=3. But 
let's try with small tables (100k rows instead of 1M rows):
                                  QUERY PLAN
---------------------------------------------------------------------------- Gather  (cost=0.29..36357.94 rows=100118
width=12)(actual 
 
time=13.219..589.723 rows=100000 loops=1)   Workers Planned: 1   Workers Launched: 1   Single Copy: true   ->  Nested
Loop (cost=0.29..36357.94 rows=100118 width=12)                    (actual time=0.288..442.821 rows=100000 loops=1)
   ->  Seq Scan on t1  (cost=0.00..1444.18 rows=100118 width=8)                      (actual time=0.148..49.308
rows=100000loops=1)         ->  Index Scan using t2_a_idx on t2                      (cost=0.29..0.34 rows=1 width=8)
                  (actual time=0.002..0.002 rows=1 loops=100000)               Index Cond: (a = t1.a) Planning time:
0.483ms Execution time: 648.941 ms
 
(10 rows)

Suddenly, we get nworkers=1 with loops=1 (and not nworkers+1 as before). 
FWIW I've only seen this with force_parallel_mode=on, and the row counts 
are correct, so perhaps that's OK. single_copy seems a bit 
underdocumented, though.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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