从PostgreSQL 9.0开始，您可以执行：

EXPLAIN (ANALYZE ON, BUFFERS ON) SELECT ...

它会告诉你语句是如何与PostgreSQL的缓存交互的。如果报告缓存缺失，那就是操作系统调用来读取一些东西。你不能确定这是物理I/O，因为它可能在操作系统缓存中。但这可能更像你在这里寻找的，而不是试图查看pg_stat_* 信息。

这个答案与特定的查询语句没有直接关系，而是为了帮助那些在这里结束的人寻找一种显示“磁盘与缓存”的方法：

-- perform a "select pg_stat_reset();" when you want to reset counter statistics
with 
all_tables as
(
SELECT  *
FROM    (
    SELECT  'all'::text as table_name, 
        sum( (coalesce(heap_blks_read,0) + coalesce(idx_blks_read,0) + coalesce(toast_blks_read,0) + coalesce(tidx_blks_read,0)) ) as from_disk, 
        sum( (coalesce(heap_blks_hit,0)  + coalesce(idx_blks_hit,0)  + coalesce(toast_blks_hit,0)  + coalesce(tidx_blks_hit,0))  ) as from_cache    
    FROM    pg_statio_all_tables  --> change to pg_statio_USER_tables if you want to check only user tables (excluding postgres's own tables)
    ) a
WHERE   (from_disk + from_cache) > 0 -- discard tables without hits
),
tables as 
(
SELECT  *
FROM    (
    SELECT  relname as table_name, 
        ( (coalesce(heap_blks_read,0) + coalesce(idx_blks_read,0) + coalesce(toast_blks_read,0) + coalesce(tidx_blks_read,0)) ) as from_disk, 
        ( (coalesce(heap_blks_hit,0)  + coalesce(idx_blks_hit,0)  + coalesce(toast_blks_hit,0)  + coalesce(tidx_blks_hit,0))  ) as from_cache    
    FROM    pg_statio_all_tables --> change to pg_statio_USER_tables if you want to check only user tables (excluding postgres's own tables)
    ) a
WHERE   (from_disk + from_cache) > 0 -- discard tables without hits
)
SELECT  table_name as "table name",
    from_disk as "disk hits",
    round((from_disk::numeric / (from_disk + from_cache)::numeric)*100.0,2) as "% disk hits",
    round((from_cache::numeric / (from_disk + from_cache)::numeric)*100.0,2) as "% cache hits",
    (from_disk + from_cache) as "total hits"
FROM    (SELECT * FROM all_tables UNION ALL SELECT * FROM tables) a
ORDER   BY (case when table_name = 'all' then 0 else 1 end), from_disk desc

不幸的是，PostgreSQL没有像SET STATISTICS IO ON那样简单。但是，通过pg_statio_*系统目录可以获得IO统计信息。由于数据没有限定在会话范围内，因此它并不完美，但如果您希望了解查询的效率以及在洁净室环境中的效率，它可以很好地解决大多数问题。
http://www.postgresql.org/docs/current/static/monitoring-stats.html

不完全是，因为PostgreSQL也严重依赖于操作系统缓存，它无法知道那里发生了什么。pg_catalog中的pg_statio* 视图家族保持着命中和实际读取的计数，但这些读取可能已经命中了操作系统缓存。