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_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;

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