这篇文章主要介绍了Postgresql 动态统计某一列的某一值出现的次数实例，具有很好的参考价值，希望对大家有所帮助。一起跟随小编过来看看吧。

实例解析：

select to_char(log.date, 'yyyy-MM-dd HH24') as hour,
  log.exten, sum(case log.grade when '1' then 1 else 0 end) as "1",
  sum(case log.grade when '2' then 1 else 0 end) as "2",
  sum(case log.grade when '3' then 1 else 0 end) as "3",
  sum(case log.grade when '4' then 1 else 0 end) as "4",
  sum(case log.grade when '5' then 1 else 0 end) as "5",
  log.direction from iface_satisfaction_investigation as log
where log.date >= '2017-08-03 00:00:00' and log.date < '2017-08-04 00:00:00'
group by hour,log.exten,log.direction order by hour,log.exten,log.direction asc

to_char:用于查询时间格式化，to_char(log.date, 'yyyy-MM-dd HH24')，大致的结果是：2017-08-03 13

sum():毫无疑问是用来计算总和的。

sum(case log.grade when '1' then 1 else 0 end) 是计算什么呢？

他的意思就是：

计算grade这个列的值为1的时候有多少行，后面的sum(……)就类推了。

其他的也没有什么好讲的了

补充：PostgreSQL常用的统计信息

我就废话不多说了，大家还是直接看代码吧~

/*计算表的空间大小*/
select oid,table_schema as "模式",
 table_name as "表名",
 row_estimate::bigint as "表中的行数(估计值)",
 pg_size_pretty(total_bytes) as "总大小",
  pg_size_pretty(table_bytes) as "表大小",
 pg_size_pretty(index_bytes) as "索引大小",
  pg_size_pretty(toast_bytes) as "toast表总大小" 
from (
 select *, total_bytes-index_bytes-coalesce(toast_bytes,0) as table_bytes
  from (
   select
    c.oid,
    nspname as table_schema,
    relname as table_name,
    c.reltuples as row_estimate,
    pg_total_relation_size(c.oid) as total_bytes,
    pg_indexes_size(c.oid) as index_bytes,
    pg_total_relation_size(reltoastrelid) as toast_bytes
   from pg_class c
   left join pg唐山百度优化_namespace n on n.oid = c.relnamespace
   where relkind = 'r'
 ) t1
) t2 order by 2,3;
/*统计用户表信息*/
select
 schemaname as "模式",
 relname as "表名",
 seq_scan as "顺序扫描的次数",
 seq_tup_read as "顺序扫描获取活动行的数量",
 idx_scan as "索引扫描次数",
 idx_tup_fetch as "索引扫描获取活动行的数量",
 n_tup_ins as "累计插入的行数",
 n_tup_upd as "累计更新的行数(包含HOT 更新的行)",