performance issue with bitmap index scans on huge amounts of big jsonb documents

Hi,

i have a performance issue with bitmap index scans on huge amounts of big jsonb documents.


===== Background =====

- table with big jsonb documents
- gin index on these documents
- queries using index conditions with low selectivity


===== Example =====

select version();
> PostgreSQL 9.6.1 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-17), 64-bit

show work_mem;
> 1GB

-- setup test data
create table bitmap_scan_test as
select
i,
(select jsonb_agg(jsonb_build_object('x', i % 2, 'filler', md5(j::text))) from generate_series(0, 100) j) big_jsonb
from
generate_series(0, 100000) i;

create index on bitmap_scan_test using gin (big_jsonb);

analyze bitmap_scan_test;


--  query with bitmap scan
explain analyze
select
count(*)
from
bitmap_scan_test
where
big_jsonb @> '[{"x": 1, "filler": "cfcd208495d565ef66e7dff9f98764da"}]';

Aggregate  (cost=272.74..272.75 rows=1 width=8) (actual time=622.272..622.272 rows=1 loops=1)
  ->  Bitmap Heap Scan on bitmap_scan_test  (cost=120.78..272.49 rows=100 width=0) (actual time=16.496..617.431
rows=50000loops=1) 
        Recheck Cond: (big_jsonb @> '[{"x": 1, "filler": "cfcd208495d565ef66e7dff9f98764da"}]'::jsonb)
        Heap Blocks: exact=637
        ->  Bitmap Index Scan on bitmap_scan_test_big_jsonb_idx  (cost=0.00..120.75 rows=100 width=0) (actual
time=16.371..16.371rows=50000 loops=1) 
              Index Cond: (big_jsonb @> '[{"x": 1, "filler": "cfcd208495d565ef66e7dff9f98764da"}]'::jsonb)
Planning time: 0.106 ms
Execution time: 622.334 ms


perf top -p... shows heavy usage of pglz_decompress:

Overhead  Shared Object     Symbol
  51,06%  postgres          [.] pglz_decompress
   7,33%  libc-2.12.so      [.] memcpy
...

===== End of example =====


I wonder why bitmap heap scan adds such a big amount of time on top of the plain bitmap index scan.
It seems to me, that the recheck is active although all blocks are exact [1] and that pg is loading the jsonb for the
recheck.

Is this an expected behavior?


Regards,
Marc-Olaf


[1] (http://dba.stackexchange.com/questions/106264/recheck-cond-line-in-query-plans-with-a-bitmap-index-scan)