Re: [9.2devel] why it doesn't do index scan only?

On 8 October 2011 18:53, Pavel Stehule <pavel.stehule@gmail.com> wrote:
> Hello
>
> 2011/10/8 Tom Lane <tgl@sss.pgh.pa.us>:
>> hubert depesz lubaczewski <depesz@depesz.com> writes:
>>> it is selecting 20 rows out of 30 million. why is it:
>>> 1. not using index only scan
>>> 2. not using even normal index scan?
>>
>> It thinks the bitmap scan is cheaper.  Whether that's true or not is not
>> very clear, but nobody is claiming that the costing of index-only scans
>> is accurate yet.
>>
>
> I did a few tests and bitmap scan is faster. Maybe there is a some
> issue. In very simple test (and very syntetic test)
>
> create table omega(a int);
> insert into omega select (random()*10000)::int from generate_series(1,400000);
>
> select count(*) from omega where a = 100;
>
> and index scan is faster than index only scan. There is lot of
> duplicates. When I used a bigger range, a speed of bitmap index, index
> only scan and index scan is similar - but index scan was faster
> everywhere.

Here, index-only scan is massively faster than any other scan:

test=# explain analyse select count(thing) from stuff where thing = 14;

QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=99814.38..99814.39 rows=1 width=4) (actual
time=337.506..337.506 rows=1 loops=1)
   ->  Index Only Scan using idx_stuff_thing on stuff
(cost=0.00..99336.88 rows=191000 width=4) (actual
time=155.955..315.106 rows=196828 loops=1)
         Index Cond: (thing = 14)
 Total runtime: 337.639 ms
(4 rows)

test=# set enable_indexonlyscan to false;
SET
test=# explain analyse select count(thing) from stuff where thing = 14;

QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=99814.38..99814.39 rows=1 width=4) (actual
time=164882.528..164882.528 rows=1 loops=1)
   ->  Index Scan using idx_stuff_thing on stuff  (cost=0.00..99336.88
rows=191000 width=4) (actual time=0.184..164494.806 rows=196828
loops=1)
         Index Cond: (thing = 14)
 Total runtime: 164882.666 ms
(4 rows)

test=# set enable_indexonlyscan to false;
SET
test=# set enable_indexscan to false;
SET
test=# explain analyse select count(thing) from stuff where thing = 14;

QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=170553.91..170553.92 rows=1 width=4) (actual
time=154102.221..154102.222 rows=1 loops=1)
   ->  Bitmap Heap Scan on stuff  (cost=2004.91..170076.41 rows=191000
width=4) (actual time=482.974..153730.892 rows=196828 loops=1)
         Recheck Cond: (thing = 14)
         ->  Bitmap Index Scan on idx_stuff_thing  (cost=0.00..1957.16
rows=191000 width=0) (actual time=421.854..421.854 rows=196828
loops=1)
               Index Cond: (thing = 14)
 Total runtime: 154107.415 ms
(6 rows)

test=# set enable_indexonlyscan to false;
SET
test=# set enable_indexscan to false;
SET
test=# set enable_bitmapscan to false;
SET
test=# explain analyse select count(thing) from stuff where thing = 14;
                                                        QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=812977.50..812977.51 rows=1 width=4) (actual
time=121296.897..121296.897 rows=1 loops=1)
   ->  Seq Scan on stuff  (cost=0.00..812500.00 rows=191000 width=4)
(actual time=67.105..121215.296 rows=196828 loops=1)
         Filter: (thing = 14)
         Rows Removed by Filter: 14803172
 Total runtime: 121296.999 ms
(5 rows)

Note: buffer cache cleared between queries.

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Thom Brown
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