Postgres 中的查询读取了多少磁盘页面？

Question

我想知道运行单个 Postgres 查询时从磁盘（而不是从缓存）读取了多少页（表 + 索引，如果有）。如果有任何方法可以从 EXPLAIN ANALYZE 中提取此信息就更好了。

Answer 1

当您添加选项时，该信息可用buffers：explain (analyze, buffers) select ...

例如

explain (analyze, buffers)
select *
from salaries s
  join employees e on e.emp_no = s.emp_no
where s.emp_no in ('10001', '20001', '30001', '40001', '50001', '99999', '99996');

QUERY PLAN                                                                                                                         
-----------------------------------------------------------------------------------------------------------------------------------
Nested Loop  (cost=0.85..1016.67 rows=81 width=51) (actual time=0.152..18.530 rows=96 loops=1)                                     
  Buffers: shared hit=407 read=5                                                                                                   
  I/O Timings: read=15.340                                                                                                         
  ->  Index Scan using salaries_pkey on salaries s  (cost=0.43..349.03 rows=81 width=20) (actual time=0.082..0.332 rows=96 loops=1)
        Index Cond: ((emp_no)::text = ANY ('{10001,20001,30001,40001,50001,99999,99996}'::text[]))                                 
        Buffers: shared hit=28                                                                                                     
  ->  Index Scan using employees_pkey on employees e  (cost=0.42..8.24 rows=1 width=31) (actual time=0.187..0.187 rows=1 loops=96) 
        Index Cond: ((emp_no)::text = (s.emp_no)::text)                                                                            
        Buffers: shared hit=379 read=5                                                                                             
        I/O Timings: read=15.340                                                                                                   
Planning Time: 256.640 ms                                                                                                          
Execution Time: 18.628 ms                                                                                                          

您可以看到总共需要 412 个页面（=块）。其中 5 个必须从文件系统中获取（“read=5”）——这 5 个是因为索引扫描而需要的employees_pkey