查找postgresql耗时最慢的SQL

    PostgreSQL部署上之后，经过一段时间的运行，我们比较关心那些SQL运行时间比较长，或者说那些SQL执行的特别慢，拖累的性能，只有找到这些SQL，才能有针对性地对这些SQL进行优化，提升PostgreSQL的性能。
   PostgreSQL提供了pg_stat_statements来存储SQL的运行次数，总运行时间，shared_buffer命中次数，shared_buffer read次数等统计信息。

Name	Type	References	Description
userid	oid	pg_authid.oid	OID of user who executed the statement
dbid	oid	pg_database.oid	OID of database in which the statement was executed
query	text		Text of the statement (up to track_activity_query_size bytes)
calls	bigint		Number of times executed
total_time	double precision		Total time spent in the statement, in seconds
rows	bigint		Total number of rows retrieved or affected by the statement
shared_blks_hit	bigint		Total number of shared blocks hits by the statement
shared_blks_read	bigint		Total number of shared blocks reads by the statement
shared_blks_written	bigint		Total number of shared blocks writes by the statement
local_blks_hit	bigint		Total number of local blocks hits by the statement
local_blks_read	bigint		Total number of local blocks reads by the statement
local_blks_written	bigint		Total number of local blocks writes by the statement
temp_blks_read	bigint		Total number of temp blocks reads by the statement
temp_blks_written	bigint		Total number of temp blocks writes by the statemen

   上图表来自PostgreSQL官方文档，注意的一点是，我的PostgreSQL是9.1.9,此时total_time的单位是秒，我观9.2的PostgreSQL的文档，total_time单位已经是毫秒。所以我的参考文献More On PostgreSQL perform 里面应该用的是9.2,因为外国这位大神默认单位是毫秒。
   可以看出，pg_stat_statements统计了SQL的很多信息，方便我们分析SQL的性能。但是这个属于PostgreSQL的扩展，需要修改postgresql.conf，才能使用：
操作步骤如下
    1 修改配置文件，并且重启PostgreSQL方能生效

1. #------------------------------------------------------------------------------
2. # PG_STAT_STATEMENTS OPTIONS
3. #------------------------------------------------------------------------------
4. shared_preload_libraries = 'pg_stat_statements'
5. custom_variable_classes = 'pg_stat_statements'
6. pg_stat_statements.max = 1000
7. pg_stat_statements.track = all

    2 创建pg_stat_statements扩展

1. CREATE EXTENSION pg_stat_statements;

    从此之后，PostgreSQL就能记录SQL的统计信息。
   上面的表格虽然丰富，其实我们基本比较关心执行最慢的SQL，如何查看执行最慢的10条SQL？

SELECT  query, calls, total_time, (total_time/calls) as average ,rows, 
        100.0 * shared_blks_hit /nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent 
FROM    pg_stat_statements 
ORDER   BY average DESC LIMIT 10;

    我在我本地的DB，查找最慢的2条SQL，输出如下：
   
   在我另一台机器上，用pgadmin查看：
   
    统计结果一直都在，重启也不会清零，那么统计结果如何清零重新统计呢？执行下面SQL即可：

1. select pg_stat_statements_reset() ;

    找到最耗时的SQL，我们就能针对这些耗时的SQL，查看是否有优化的余地。

参考文献：
1 More on Postgres Performance
2   PostgreSQL manual