Postgresql 动态统计某一列的某一值出现的次数实例

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_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)",	
	n_tup_del as "累计删除的行数",
	n_live_tup as "当前活动行估计数量",
	n_dead_tup as "当前死亡行的估计数量",
	n_mod_since_analyze as "最后一次分析后被修改的行估计数量",
	last_vacuum as "上次被手动清理的时间(不统计VACUUM FULL)",
	last_autovacuum as "上次自动清理的时间",
	last_analyze as "上次手动分析的时间",
	last_autoanalyze as "上次自动清理分析的时间",
	vacuum_count as "手动清理的次数",
	autovacuum_count as "自动清理的次数",
	analyze_count as "手动分析的次数",
	autoanalyze_count as "自动分析的次数",
	pg_size_pretty(pg_table_size(relid)) as "表大小(不包含索引)"
from pg_stat_user_tables
order by 1;
/*统计用户表IO信息*/
select
	schemaname as "模式",
	relname as "表名",
	heap_blks_read as "读取的磁盘块数量",
	heap_blks_hit as "缓冲区命中数量",
	idx_blks_read as "表上所有索引读取的磁盘块数",
	idx_blks_hit as "表上的所有索引缓冲区命中数量",
	toast_blks_read as "TOAST表(如果有)读取的磁盘块数",
	toast_blks_hit as "TOAST表(如果有)缓冲区命中数量",
	tidx_blks_read as "TOAST表索引(如果有)读取的磁盘块数",
	tidx_blks_hit as "TOAST表索引(如果有)缓冲区命中数量"
from pg_statio_user_tables
order by 1;
/*统计用户索引信息*/
select 
	indexrelid,
	schemaname as "模式",
	relname as "索引所在的表名称",
	indexrelname as "索引名称",
	idx_scan as "索引扫描次数",
	idx_tup_read as "索引扫描返回的索引项数量",
	idx_tup_fetch as "简单索引扫描获取的活动行数量",
	pg_size_pretty(pg_relation_size(indexrelid)) as "索引大小"
from pg_stat_user_indexes
order by 1,2;
/*追踪函数,需要打开track_functions参数(默认关闭)*/
select * from pg_stat_user_functions;

以上为个人经验，希望能给大家一个参考，也希望大家多多支持。如有错误或未考虑完全的地方，望不吝赐教。