Re: Use of index for 50% column restriction

On Wed, Jun  8, 2016 at 05:07:34PM -0400, Bruce Momjian wrote:
> > For randomly-ordered data I believe the cutover is actually well below 10%.
> 
> Ah, I had not considered the correlation order of the rows in the table.
> This test returns the sequential scan I expected by using floor(random()
> * 2):
> 
>     DROP TABLE IF EXISTS test;
>     CREATE TABLE test (c1 INT, c2 INT, c3 INT);
>     INSERT INTO test SELECT c1, floor(random() * 2), 0 FROM generate_series(1, 10000) AS a(c1);
>     INSERT INTO test SELECT c1, floor(random() * 2), 1 FROM generate_series(10001, 20000) AS a(c1);
>     CREATE INDEX i_test_c2 ON test (c2);
>     ANALYZE test;
>     EXPLAIN SELECT * FROM test WHERE c2 = 0;
> 
> Thanks.

Just a follow-up, but even with a randomized correlation order, it seems
25% restrictivity generates a Bitmap Index Scan:
DROP TABLE IF EXISTS test;CREATE TABLE test (c1 INT, c2 INT, c3 INT);INSERT INTO test SELECT c1, abs(floor(random() *
4)-1),abs(floor(random() * 4)-1) FROM generate_series(1, 10000) AS a(c1);INSERT INTO test SELECT c1, abs(floor(random()
*4)-1), abs(floor(random() * 4)-1) FROM generate_series(10001, 15000) AS a(c1);INSERT INTO test SELECT c1,
abs(floor(random()* 4)-1), abs(floor(random() * 4)-1) FROM generate_series(15001, 20000) AS a(c1);CREATE INDEX
i_test_c2ON test (c2);ANALYZE test;
 
SELECT c2, COUNT(*) FROM test GROUP BY c2 ORDER BY 1; c2 | count----+-------  0 |  5020  25%  1 | 10006  50%  2 |  4974
25%
 
EXPLAIN SELECT * FROM TEST WHERE c2 = 1;                            QUERY
PLAN-----------------------------------------------------------Seq Scan on test  (cost=0.00..359.00 rows=10006
width=12)  Filter: (c2 = 1)
 
EXPLAIN SELECT * FROM TEST WHERE c2 = 0;                                 QUERY
PLAN----------------------------------------------------------------------------Bitmap Heap Scan on test
(cost=99.19..270.94rows=5020 width=12)   Recheck Cond: (c2 = 0)   ->  Bitmap Index Scan on i_test_c2  (cost=0.00..97.94
rows=5020width=0)         Index Cond: (c2 = 0)
 

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com

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