On 03/12/2017 01:44, Tom Lane wrote:
I think it'd be a serious error to screw around with your cost settings
on the basis of a single case in which the rowcount estimates are so
far off.  It's really those estimates that are the problem AFAICS.

The core issue in this example is that, the way the test data is set up,
the "flag = true" condition actually adds no selectivity at all, because
every row with "num = 1" is certain to have "flag = true".  If the planner
realized that, it would certainly not bother with BitmapAnd'ing the flag
index onto the results of the num index.  But it doesn't know that those
columns are correlated, so it supposes that adding the extra index will
give a 10x reduction in the number of heap rows that have to be visited
(since it knows that only 1/10th of the rows have "flag = true").
*That* is what causes the overly optimistic cost estimate for the
two-index bitmapscan, and no amount of fiddling with the cost parameters
will make that better.
Here I've tried to make a test which would not have correlation between the two columns.

shared_buffers = 512MB
effective_cache_size = 512MB
work_mem = 100MB

set seq_page_cost = 1.0;
set random_page_cost = 1.5;
set cpu_tuple_cost = 0.01;
set cpu_index_tuple_cost = 0.005;
set cpu_operator_cost = 0.0025;

drop table if exists aaa;
create table aaa as select floor(random()*100)::int num, (random()*10 < 1)::bool flag from generate_series(1, 10000000) id;
create index i1 on aaa  (num);
create index i2 on aaa  (flag);
analyze aaa;

select relname, reltuples::bigint, relpages::bigint, (reltuples/relpages)::bigint tpp from pg_class where relname in('aaa','i1','i2') order by relname;

select flag, count(*) from aaa group by flag order by flag;

select num, count(*) from aaa group by num order by num;

explain (analyze,verbose,costs,buffers)
select * from aaa where num = 1 and flag = true;

Bitmap Heap Scan on public.aaa  (cost=12829.83..24729.85 rows=10340 width=5) (actual time=104.941..112.649 rows=9944 loops=1)
  Output: num, flag
  Recheck Cond: (aaa.num = 1)
  Filter: aaa.flag
  Heap Blocks: exact=8922
  Buffers: shared hit=11932
  ->  BitmapAnd  (cost=12829.83..12829.83 rows=10340 width=0) (actual time=102.926..102.926 rows=0 loops=1)
        Buffers: shared hit=3010
        ->  Bitmap Index Scan on i1  (cost=0.00..1201.44 rows=103334 width=0) (actual time=15.459..15.459 rows=99631 loops=1)
              Index Cond: (aaa.num = 1)
              Buffers: shared hit=276
        ->  Bitmap Index Scan on i2  (cost=0.00..11622.97 rows=1000671 width=0) (actual time=76.906..76.906 rows=999339 loops=1)
              Index Cond: (aaa.flag = true)
              Buffers: shared hit=2734
Planning time: 0.110 ms
Execution time: 113.272 ms

Index Scan using i1 on public.aaa  (cost=0.44..66621.56 rows=10340 width=5) (actual time=0.027..47.075 rows=9944 loops=1)
  Output: num, flag
  Index Cond: (aaa.num = 1)
  Filter: aaa.flag
  Rows Removed by Filter: 89687
  Buffers: shared hit=39949
Planning time: 0.104 ms
Execution time: 47.351 ms

I tried creating multiple-column statistics using the v10 facility for

regression=# create statistics s1 on num, flag from aaa;
regression=# analyze aaa;

but that changed the estimate not at all, which surprised me because
dependency statistics are supposed to fix exactly this type of problem.
I suspect there may be something in the extended-stats code that causes it
not to work right for boolean columns --- this wouldn't be excessively
surprising because of the way the planner messes around with converting
"flag = true" to just "flag" and sometimes back again.  But I've not
looked closer yet.

                        regards, tom lane

Reply via email to