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假设我有一个查询返回属性 R:a 具有第二高的不同值（返回表名和属性名）。我的任务是：

对于相同属性 R:a 的每个不同值，将不同值返回为 val，并将 R 中具有此值的元组数返回为计数。显示这些不同值的结果，例如 20% 的 R 元组具有等于或低于 R:a 值，20% 的 R 元组具有更高或等于 R:a 值。

建议对存储进行修改，以便查询尽可能高效地运行。

我编写了一个查询来返回不同的值和计数，但是我如何显示 20% 的结果以及在这种情况下存储修改意味着什么？

SELECT pg_stats.tablename,pg_stats.attname,UNNEST(pg_stats.most_common_vals::text::varchar[]) as val, UNNEST(pg_stats.most_common_freqs)*pg_class.reltuples as count
FROM information_schema.columns,pg_stats,pg_class, 

( SELECT T.tablename,T.attname
FROM (
SELECT pg_stats.tablename, pg_stats.attname, pg_stats.n_distinct as distval
FROM information_schema.columns,pg_stats
WHERE pg_stats.tablename = table_name AND pg_stats.attname = column_name AND (NOT (information_schema.columns.table_schema LIKE 'pg_%' OR information_schema.columns.table_schema = 'information_schema')) AND (information_schema.columns.data_type ='character varying') AND (table_name NOT in (SELECT viewname FROM pg_views))
LIMIT 5) as T
ORDER by T.distval DESC limit 1 offset 1) as temp 


WHERE (pg_stats.tablename = table_name AND pg_stats.attname = column_name AND (NOT 
(information_schema.columns.table_schema LIKE 'pg_%' OR 
information_schema.columns.table_schema 
  = 'information_schema')) AND (information_schema.columns.data_type ='character varying') AND 
  (table_name NOT in (SELECT viewname FROM pg_views))
  AND pg_stats.tablename = relname AND temp.tablename = pg_stats.tablename AND temp.attname = 
  pg_stats.attname);
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