Postgresql postgres CASE和where子句

假设我有一张像这样的桌子：

id  item  type  status  date
1   A     1     P       2018-04-01
2   B     2     A       2018-01-01
3   C     1     A       2018-01-02 
4   D     2     A       2018-04-11

现在查询1：

Select count(case when type=1 and status='A' and date<'2018-04-01' then id) as type1, 
count(case when type=2 and status='A' and date<'2018-04-01' then id)as type2
FROM table

和问题2：

Select count(case when type=1 then id) as type1, 
    count(case when type=2 then id)as type2 
FROM table where status='A' and date < '2018-04-01' 

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这两个版本是否相同，如果是whcih更好如果日期列上有索引，那么第二个版本可能会比第一个版本更好。这样做的原因是，Postgres甚至可以在SELECT子句中进行条件聚合之前过滤掉WHERE中的许多记录

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如果date没有索引，则无论哪种情况，Postgres都必须接触表中的每条记录。

使用如下模式：

第二个查询执行得更好，原因是：

Rows Removed by Filter: 2

<>但它们都不使用任何索引，对于大量数据，您应该考虑添加新索引并再次检查性能。

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dbfiddle

请添加当前的表架构。

Rows Removed by Filter: 2

explain (analyze,buffers)
select count(case when type = 1 and status = 'A' and date < '2018-04-01' then id end) as type1, 
       count(case when type = 2 and status = 'A' and date < '2018-04-01' then id end) as type2
from   tbl;

| QUERY PLAN | | :---------------------------------------------------------------------------------------------------- | | Aggregate (cost=33.40..33.41 rows=1 width=16) (actual time=0.022..0.022 rows=1 loops=1) | | Buffers: shared hit=1 | | -> Seq Scan on tbl (cost=0.00..17.80 rows=780 width=44) (actual time=0.006..0.007 rows=4 loops=1) | | Buffers: shared hit=1 | | Planning time: 0.171 ms | | Execution time: 0.121 ms |

explain (analyze,buffers)
select count(case when type=1 then id end) as type1, 
       count(case when type=2 then id end)as type2 
from   tbl 
where  status = 'A' 
and    date < '2018-04-01';

| QUERY PLAN | | :------------------------------------------------------------------------------------------------- | | Aggregate (cost=21.71..21.72 rows=1 width=16) (actual time=0.007..0.008 rows=1 loops=1) | | Buffers: shared hit=1 | | -> Seq Scan on tbl (cost=0.00..21.70 rows=1 width=8) (actual time=0.004..0.005 rows=2 loops=1) | | Filter: ((date < '2018-04-01'::date) AND (status = 'A'::text)) | | Rows Removed by Filter: 2 | | Buffers: shared hit=1 | | Planning time: 0.084 ms | | Execution time: 0.032 ms |