PostgreSQL之查找最慢的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	.oid	OID of user who executed the statement
dbid	oid	.oid	OID of database in which the statement was executed
query	text		Text of the statement (up to  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 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

#------------------------------------------------------------------------------
# PG_STAT_STATEMENTS OPTIONS
#------------------------------------------------------------------------------
shared_preload_libraries = 'pg_stat_statements'
custom_variable_classes = 'pg_stat_statements'
pg_stat_statements.max = 1000
pg_stat_statements.track = all

    2 创建pg_stat_statements扩展

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：

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 2   PostgreSQL manual