问题描述
当我通过 agreement_id 运行查询和过滤时，它很慢，
但是当我按别名 id 过滤时，它很快。 （看查询的结尾）
为什么过滤时使用相同的字段会导致不同的执行时间？
指向 explain analyze 的链接：
差异从#20 开始：使用不同索引的地方：Index Cond: (o.sys_period @> sys_time()) VS Index Cond: (o.agreement_id = 38)
附注。如果我能联系到此功能的开发者就好了（我还有一个类似的问题）
UPD 我做了一些实验。当我从查询中删除窗口函数时，它在任何情况下都可以快速运行。那么为什么窗口函数在某些情况下会停止使用索引？如何逃避/解决这个问题？
dbfiddle 使用最少的测试用例
服务器版本为 v13.1
完整查询：
WITH gconf AS
-- https://www.postgresql.org/docs/current/queries-with.html#QUERIES-WITH-SELECT
NOT MATERIALIZED -- force it to be merged into the parent query
-- it gives a net savings because each usage of the WITH query needs only a small part of the WITH query's full output.
( SELECT
ocd.*,tstzrange( '2021-05-01','2021-05-01','[]') AS acc_period,(o).agreement_id AS id,-- required to passthrough WINDOW FUNCTION
(o).id AS order_id,(ic).consumed_period AS consumed_period,dense_rank() OVER ( PARTITION BY (o).agreement_id,(o).id ORDER BY (ic).consumed_period ) AS nconf,row_number() OVER ( wconf ORDER BY (c).sort_order NULLS LAST ) AS nitem,(sum( ocd.item_cost ) OVER wconf)::numeric( 10,2) AS conf_cost,max((ocd.ic).consumed) OVER wconf AS consumed,CASE WHEN true
THEN (sum( ocd.item_suma ) OVER wconf)::numeric( 10,2 )
ELSE (sum( ocd.item_cost ) OVER wconf)::numeric( 10,2 )
END AS conf_suma
FROM order_cost_details( tstzrange( '2021-05-01','[]') ) ocd
WHERE true OR (ocd.ic).consumed_period @> lower( tstzrange( '2021-05-01','[]') )
WINDOW wconf AS ( PARTITION BY (o).agreement_id,(o).id,(ic).consumed_period )
),gorder AS (
SELECT *,(conf_suma/6)::numeric( 10,2 ) as conf_nds,sum( conf_suma ) FILTER (WHERE nitem = 1) OVER worder AS order_suma
FROM gconf
WINDOW worder AS ( PARTITION BY gconf.id,(o).id )
-- Todo: Ask PG developers: Why changing to (o).agreement_id slows down query?
-- WINDOW worder AS ( PARTITION BY (o).agreement_id,(o).id )
)
SELECT
u.id,consumed_period,nconf,nitem,(c).id as item_id,COALESCE( (c).sort_order,pd.sort_order ) as item_order,COALESCE( st.display,st.name,rt.display,rt.name ) as item_name,COALESCE( item_qty,(c).amount/rt.unit ) as item_qty,COALESCE( (p).label,rt.label ) as measure,item_price,item_cost,item_suma,conf_cost,consumed,conf_suma,conf_nds,order_suma,(order_suma/6)::numeric( 10,2 ) as order_nds,sum( conf_suma ) FILTER (WHERE nitem = 1 ) OVER wagreement AS total_suma,sum( (order_suma/6)::numeric( 10,2 ) ) FILTER (WHERE nitem = 1 AND nconf = 1) OVER wagreement AS total_nds,pkg.id as package_id,pkg.link_1c_id as package_1c_id,COALESCE( pkg.display,pkg.name ) as package,acc_period
FROM gorder u
LEFT JOIN resource_type rt ON rt.id = (c).resource_type_id
LEFT JOIN service_type st ON st.id = (c).service_type_id
LEFT JOIN package pkg ON pkg.id = (o).package_id
LEFT JOIN package_detail pd ON pd.package_id = (o).package_id
AND pd.resource_type_id IS NOT disTINCT FROM (c).resource_type_id
AND pd.service_type_id IS NOT disTINCT FROM (c).service_type_id
-- WHERE (o).agreement_id = 38 -- slow
WHERE u.id = 38 -- fast
WINDOW wagreement AS ( PARTITION BY (o).agreement_id )
解决方法
作为解决问题的方法，我们还可以为 SELECT 表达式中使用的列添加别名 PARTITION BY。然后PG应用优化并使用索引。
问题的答案可能是：如果使用复合类型，PG 不会应用优化。请注意它的工作原理：
PARTITION | FILTER | IS USED?
------------------------------
ALIAS | ORIG | NO
ALIAS | ALIAS | YES
ORIG | ALIAS | NO
ORIG | ORIG | NO
create table agreement ( ag_id int,name text,cost numeric(10,2) );
create index ag_idx on agreement (ag_id);
insert into agreement (ag_id,name,cost) values ( 1,'333',22 ),(1,33),7),(2,'555',18 ),2),(3,'777',4);
select * from agreement;
create function initial ()
returns table( agreement_id int,ag agreement ) language sql stable AS $$
select ag_id,t from agreement t;
$$;
select * from initial() t;
explain( analyze,costs,buffers,verbose ) with totals_by_ag as (
select
*,sum( (t.ag).cost ) over ( partition by agreement_id ) as total
from initial() t
)
select * from totals_by_ag t
where (t.ag).ag_id = 1; -- index is NOT USED
explain( analyze,sum( (t.ag).cost ) over ( partition by agreement_id ) as total
from initial() t
)
select * from totals_by_ag t
where agreement_id = 1; -- index is used when alias for column is used
explain( analyze,sum( (t.ag).cost ) over ( partition by (t.ag).ag_id ) as total --renamed
from initial() t
)
select * from totals_by_ag t
where agreement_id = 1; -- index is NOT USED because grouping by original column
explain( analyze,sum( (t.ag).cost ) over ( partition by (t.ag).ag_id ) as total --renamed
from initial() t
)
select * from totals_by_ag t
where (t.ag).ag_id = 1; -- index is NOT USED even if at both cases original column