实例解析：
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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
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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~
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/*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_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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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服务器之家。如有错误或未考虑完全的地方，望不吝赐教。
原文链接：https://blog.csdn.net/qq_34090008/article/details/76618335