Re: Use of index for 50% column restriction

On Wed, Jun 8, 2016 at 01:28:54PM -0400, Tom Lane wrote:
> Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > As part of my research on the parsing/planning behavior of PREPARE, I
> > found a surprising behavior --- a WHERE clause that is 50% restrictive
> > is using an index. I thought only <10% restrictions used indexes.
>
> There's no such hard-and-fast rule. The cost estimate break point depends
> greatly on the index order correlation (which is 100% in your example),
> as well as some other factors like the index size versus
> effective_cache_size.
>
> 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.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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