1、统计最新数据

analyze t;

2、使用 pg_stats 查询字段唯一值数量
select attname, n_distinct
from pg_stats
where tablename = ‘t’;

• n_distinct>1，表示唯一值的估算数量。
• n_distinct<1，表示唯一值占比，例如-1，意味着几乎所有值都是唯一（适合索引）

3、直接计算真实唯一值数量

Select
COUNT(DISTINCT a) AS a_distinct,
COUNT(DISTINCT b) AS b_distinct,
COUNT(DISTINCT c) AS c_distinct
FROM t;