计算不同的值?

它不会比以下更简单:

SELECT count(DISTINCT a) ...

上的索引 (a) 无能为力。

对于中基数较低的大表 a (许多重复),模拟索引跳过扫描的性能(要好得多)。喜欢:

WITH RECURSIVE cte AS (
   (   -- parentheses required
   SELECT a
   FROM   tbl
   ORDER  BY 1
   LIMIT  1
   )
   UNION ALL
   SELECT l.*
   FROM   cte c
   CROSS  JOIN LATERAL (
      SELECT a
      FROM   tbl t
      WHERE  t.a > c.a
      ORDER  BY 1
      LIMIT  1
      ) l
   )
SELECT count(*) FROM cte;

当然不是“更简单”。并且需要索引支持。
请参阅:

• SELECT DISTINCT is slower than expected on my table in PostgreSQL
• Optimize GROUP BY query to retrieve latest row per user