Question

在这个execution plan中，以下查询花费了大量时间（3.780秒）来仅对order_line表执行索引扫描（执行计划的第11行）。

表的主键：

Customer PK - c_w_id, c_d_id, c_id
OOrder PK - o_w_id, o_d_id, o_id
Order_line PK - ol_w_id, ol_d_id, ol_o_id, ol_number
Nation PK - n_nationkey

查询：

select   c_id, c_last, sum(ol_amount) as revenue, c_city, c_phone, n_name
from     customer, oorder, order_line, nation
where    c_id = o_c_id
     and c_w_id = o_w_id
     and c_d_id = o_d_id
     and ol_w_id = o_w_id
     and ol_d_id = o_d_id
     and ol_o_id = o_id
     and o_entry_d >= '2007-01-02 00:00:00.000000'
     and o_entry_d <= ol_delivery_d
     and n_nationkey = ascii(substr(c_state,1,1))
group by c_id, c_last, c_city, c_phone, n_name
order by revenue desc

如何改善此查询的效果？你推荐什么物化观点？这是一个不错的选择吗？

CREATE MATERIALIZED VIEW mview AS
select c_id, c_last, ol_amount, c_city, c_phone, o_entry_d, ol_delivery_d, c_state
from     customer, oorder, order_line
where    c_id = o_c_id
     and c_w_id = o_w_id
     and c_d_id = o_d_id
     and ol_w_id = o_w_id
     and ol_d_id = o_d_id
     and ol_o_id = o_id;

Answer 1

切勿使用此类列名。
如果不是1）那么永远不要在查询中写入没有表前缀的列名（不需要在列名中添加前缀）。
最好在＆＃34; join＆＃34;中编写联接。条款，不在＆＃34;其中＆＃34;因此，您可以使用连接的层次结构进行操作。

最后，你运行

analyse customer; 
analyse order_line; 
analyse oorder; 
analyse nation;

在ascii（substr（c_state，1,1））上添加索引后？如果没有 - 运行它。

您也可以在select子句中将n_name作为子查询：

select c_id, c_last, revenue, c_city, c_phone, (select x.n_name from nation x where x.n_nationkey = ascii(c_statecut)) as n_name 
  from (
select   c_id, c_last, sum(ol_amount) as revenue, c_city, c_phone, substr(c_state,1,1) as c_statecut
from     customer, oorder, order_line
where    c_id = o_c_id
     and c_w_id = o_w_id
     and c_d_id = o_d_id
     and ol_w_id = o_w_id
     and ol_d_id = o_d_id
     and ol_o_id = o_id
     and o_entry_d >= '2007-01-02 00:00:00.000000'
     and o_entry_d <= ol_delivery_d
group by c_id, c_last, c_city, c_phone, c_statecut
) finale
order by revenue desc

Nation表是一个简单的查找表，但它会影响聚合的整体计划。

关于物化视图，仅对order_line表进行预聚合是获得总计的最快方法，但就你而言，＆＃34; o_entry_d＆lt; = ol_delivery_d＆＃34;在查询条件中，您无法单独使用order_line表来计算聚合。尝试将其作为物化（和索引）视图，以便稍后与客户和国家联合。

select o_c_id, ol_w_id, ol_d_id, ol_o_id, sum(ol_amount)
  from order_line 
  join oorder
    on     ol_w_id = o_w_id
       and ol_d_id = o_d_id
       and ol_o_id = o_id
       and o_entry_d <= ol_delivery_d -- (very strange condition, which looks redundant... it could be better if you can remove it)
 where o_entry_d >= '2007-01-02 00:00:00.000000'
 group by o_c_id, ol_w_id, ol_d_id, ol_o_id

如果此查询大约需要3秒钟，那么实现它。如果少于3秒，那么您可以在运行中与其他人一起加入。

Answer 2

您可以尝试使用ascii(substr(c_state,1,1))的表达式索引。您对客户进行了顺序扫描。

如何在索引扫描中提高PostgreSQL查询的性能

2 个答案: