小编Sha*_*ane的帖子

考虑以下示例：

CREATE TABLE test (
  id SERIAL,
  some_integer INT
);

INSERT INTO test (some_integer)
SELECT FLOOR(RANDOM()*100000) from generate_series(1,100000) s(i);

CREATE INDEX some_integer_idx ON test (some_integer);

EXPLAIN ANALYZE SELECT COUNT(DISTINCT some_integer) from test;

它返回以下查询计划：

CREATE TABLE test (
  id SERIAL,
  some_integer INT
);

INSERT INTO test (some_integer)
SELECT FLOOR(RANDOM()*100000) from generate_series(1,100000) s(i);

CREATE INDEX some_integer_idx ON test (some_integer);

EXPLAIN ANALYZE SELECT COUNT(DISTINCT some_integer) from test;

我很惊讶它仍然在测试中进行顺序扫描。简单地计算索引中的行数不是更快吗？