Use of index for 50% column restriction

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. To
setup the test:

DROP TABLE IF EXISTS test;
CREATE TABLE test (c1 INT, c2 INT, c3 INT);
INSERT INTO test SELECT c1, 0, 0 FROM generate_series(1, 10000) AS a(c1);
INSERT INTO test SELECT c1, 1, 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;

The output is:

QUERY PLAN
-----------------------------------------------------------------------------
Index Scan using i_test_c2 on test (cost=0.29..349.29 rows=10000 width=12)
----------
Index Cond: (c2 = 0)
(2 rows)

\timing does show the optimizer is making the right decision to use the
index, and this behavior is the same back to at least 9.3. Setting
effective_cache_size = '8kB' does not change this behavior. What am I
missing? Is my 10% assumption wrong?

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

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