我正在尝试使用包含我需要的数据的两个表进行查询。下面有3个查询。第一个是我需要的第一个表的查询。第二个是我需要的第二个表的查询。第三个是我需要的两个表的查询。
以下是我的查询：
/*Query with including the first table I need*/
set schema 'exampledata';
select
count(signature),
signature,
example1
from testtable
group by testtable.signature, testtable.example1
order by count(signature) desc
/*Row count: 1852 rows
Query time: 2.1 secs*/
/*Query with including the second table I need*/
set schema 'exampledata';
select
count(signature),
signature,
example2
from testtable
group by testtable.signature, testtable.example2
order by count(signature) desc
/*Row count: 1794 rows
Query time: 2.1 secs*/
/*Query with including the first and second tables I need*/
set schema 'exampledata';
select
count(signature),
signature,
example2,
example1
from testtable
group by testtable.signature, testtable.example2, testtable.example1
order by count(signature) desc
/*Row count: 1872 rows
Query time: 23.5 secs*/
为什么一旦我向查询添加一个表，处理所需的时间增加了十倍？是因为表没有共享相同数量的行吗？
以下是解释，尊重所显示的每个查询：
"Sort (cost=151128.71..151149.41 rows=8280 width=79) (actual time=1563.842..1563.869 rows=1852 loops=1)"
" Output: (count(signature)), signature, example1"
" Sort Key: (count(testtable.signature)) DESC"
" Sort Method: quicksort Memory: 316kB"
" -> HashAggregate (cost=150507.08..150589.88 rows=8280 width=79) (actual time=1563.141..1563.476 rows=1852 loops=1)"
" Output: count(signature), signature, example1"
" Group Key: testtable.signature, testtable.example1"
" -> Seq Scan on data.testtable (cost=0.00..129087.90 rows=2855890 width=79) (actual time=0.005..618.993 rows=2807146 loops=1)"
" Output: example14, example3, example4, example5, example1, example2, example6, example7, example8, example9, example10, example11, example12, example13"
"Planning time: 0.063 ms"
"Execution time: 1564.021 ms"
"Sort (cost=152197.99..152249.74 rows=20700 width=85) (actual time=1584.287..1584.312 rows=1794 loops=1)"
" Output: (count(signature)), signature, example2"
" Sort Key: (count(testtable.signature)) DESC"
" Sort Method: quicksort Memory: 313kB"
" -> HashAggregate (cost=150507.08..150714.08 rows=20700 width=85) (actual time=1583.538..1583.916 rows=1794 loops=1)"
" Output: count(signature), signature, example2"
" Group Key: testtable.signature, testtable.example2"
" -> Seq Scan on data.testtable (cost=0.00..129087.90 rows=2855890 width=85) (actual time=0.005..628.403 rows=2807146 loops=1)"
" Output: example14, example3, example4, example5, example1, example2, example6, example7, example8, example9, example10, example11, example12, example13"
"Planning time: 0.059 ms"
"Execution time: 1584.542 ms"
"Sort (cost=794824.44..795341.94 rows=207000 width=92) (actual time=22771.282..22771.307 rows=1872 loops=1)"
" Output: (count(signature)), signature, example2, example1"
" Sort Key: (count(testtable.signature)) DESC"
" Sort Method: quicksort Memory: 350kB"
" -> GroupAggregate (cost=728162.97..765931.60 rows=207000 width=92) (actual time=18818.703..22770.504 rows=1872 loops=1)"
" Output: count(signature), signature, example2, example1"
" Group Key: testtable.signature, testtable.example2, testtable.example1"
" -> Sort (cost=728162.97..735302.70 rows=2855890 width=92) (actual time=18803.887..22232.128 rows=2807146 loops=1)"
" Output: signature, example2, example1"
" Sort Key: testtable.signature, testtable.example2, testtable.example1"
" Sort Method: external merge Disk: 286688kB"
" -> Seq Scan on data.testtable (cost=0.00..129087.90 rows=2855890 width=92) (actual time=0.008..960.797 rows=2807146 loops=1)"
" Output: signature, example2, example1"
"Planning time: 0.084 ms"
"Execution time: 23085.289 ms"