Re: Bitmap scan is undercosted?

[Justin Pryzby <pryzby@xxxxxxxxxxxxx>]

On Sat, Dec 02, 2017 at 01:54:09AM +0200, Vitaliy Garnashevich wrote:
> On 02/12/2017 01:11, Justin Pryzby wrote:
> >..which is what's wanted with no planner hints (PG10.1 here).
> Yes, that's what you get without planner hints, but it's strange to get this
> plan, when there is another one, which runs 2-3 times faster, but happens to
> be estimated to be twice more costly than the one with bitmap scans:
> 
> # set enable_bitmapscan = off; set enable_indexscan = on;  set enable_seqscan = off;
> # explain analyze select * from aaa where num = 1 and flag = true;
> Index Scan using i1 on aaa  (cost=0.44..66369.81 rows=10428 width=5) (actual time=0.020..57.765 rows=100000 loops=1)
> 
> vs.
> 
> # set enable_bitmapscan = on;  set enable_indexscan = off; set enable_seqscan = off;
> # explain analyze select * from aaa where num = 1 and flag = true;
> Bitmap Heap Scan on aaa  (cost=13099.33..25081.40 rows=10428 width=5) (actual time=122.137..182.811 rows=100000 loops=1)

I was able to get an index plan with:

SET random_page_cost=1; SET cpu_index_tuple_cost=.04; -- default: 0.005; see selfuncs.c
postgres=# EXPLAIN (analyze,verbose,costs,buffers) SELECT * FROM aaa WHERE num=1 AND flag=true; 
 Index Scan using i1 on public.aaa  (cost=0.43..50120.71 rows=10754 width=5) (actual time=0.040..149.580 rows=100000 loops=1)

Or with:
SET random_page_cost=1; SET cpu_operator_cost=0.03; -- default: 0.0025 see cost_bitmap_tree_node()
EXPLAIN (analyze,verbose,costs,buffers) SELECT * FROM aaa WHERE num=1 AND flag= true;  
 Index Scan using i1 on public.aaa  (cost=5.22..49328.00 rows=10754 width=5) (actual time=0.051..109.082 rows=100000 loops=1)

Or a combination trying to minimize the cost of the index scan:
postgres=# SET random_page_cost=1; SET cpu_index_tuple_cost=.0017; SET cpu_operator_cost=0.03; EXPLAIN (analyze,verbose,costs,buffers) SELECT * FROM aaa WHERE num=1 AND flag= true;  
 Index Scan using i1 on public.aaa  (cost=5.22..48977.10 rows=10754 width=5) (actual time=0.032..86.883 rows=100000 loops=1)

Not sure if that's reasonable, but maybe it helps to understand.

Justin