我试图理解两个查询性能差异如此之大的原因。
假设我有两个表格。第一个表格包含了一些域名的 A 记录:
Table "public.dns_a"
Column | Type | Modifiers | Storage | Stats target | Description
--------+------------------------+-----------+----------+--------------+-------------
name | character varying(125) | | extended | |
a | inet | | main | |
Indexes:
"dns_a_a_idx" btree (a)
"dns_a_name_idx" btree (name varchar_pattern_ops)
第二个表格处理CNAME记录:
Table "public.dns_cname"
Column | Type | Modifiers | Storage | Stats target | Description
--------+------------------------+-----------+----------+--------------+-------------
name | character varying(256) | | extended | |
cname | character varying(256) | | extended | |
Indexes:
"dns_cname_cname_idx" btree (cname varchar_pattern_ops)
"dns_cname_name_idx" btree (name varchar_pattern_ops)
现在我正在尝试解决一个“简单”的问题,即获取指向同一IP地址的所有域名,包括CNAME。
第一次尝试使用CTE效果还不错:
EXPLAIN ANALYZE WITH RECURSIVE names_traverse AS (
(
SELECT name::varchar(256), NULL::varchar(256) as cname, a FROM dns_a WHERE a = '118.145.5.20'
)
UNION ALL
SELECT c.name, c.cname, NULL::inet as a FROM names_traverse nt, dns_cname c WHERE c.cname=nt.name
)
SELECT * FROM names_traverse;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------
CTE Scan on names_traverse (cost=3051757.20..4337044.86 rows=64264383 width=1064) (actual time=0.037..1697.444 rows=199 loops=1)
CTE names_traverse
-> Recursive Union (cost=0.57..3051757.20 rows=64264383 width=45) (actual time=0.036..1697.395 rows=199 loops=1)
-> Index Scan using dns_a_a_idx on dns_a (cost=0.57..1988.89 rows=1953 width=24) (actual time=0.035..0.064 rows=14 loops=1)
Index Cond: (a = '118.145.5.20'::inet)
-> Merge Join (cost=4377.00..176448.06 rows=6426243 width=45) (actual time=498.101..848.648 rows=92 loops=2)
Merge Cond: ((c.cname)::text = (nt.name)::text)
-> Index Scan using dns_cname_cname_idx on dns_cname c (cost=0.56..69958.06 rows=2268434 width=45) (actual time=4.732..688.456 rows=2219973 loops=2)
-> Materialize (cost=4376.44..4474.09 rows=19530 width=516) (actual time=0.039..0.084 rows=187 loops=2)
-> Sort (cost=4376.44..4425.27 rows=19530 width=516) (actual time=0.037..0.053 rows=100 loops=2)
Sort Key: nt.name USING ~<~
Sort Method: quicksort Memory: 33kB
-> WorkTable Scan on names_traverse nt (cost=0.00..390.60 rows=19530 width=516) (actual time=0.001..0.007 rows=100 loops=2)
Planning time: 0.130 ms
Execution time: 1697.477 ms
(15 rows)
以上示例中有两个循环,因此如果我进行简单的外连接查询,则会得到更好的结果:
EXPLAIN ANALYZE
SELECT *
FROM dns_a a
LEFT JOIN dns_cname c1 ON (c1.cname=a.name)
LEFT JOIN dns_cname c2 ON (c2.cname=c1.name)
WHERE a.a='118.145.5.20';
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------
Nested Loop Left Join (cost=1.68..65674.19 rows=1953 width=114) (actual time=1.086..12.992 rows=189 loops=1)
-> Nested Loop Left Join (cost=1.12..46889.57 rows=1953 width=69) (actual time=1.085..2.154 rows=189 loops=1)
-> Index Scan using dns_a_a_idx on dns_a a (cost=0.57..1988.89 rows=1953 width=24) (actual time=0.022..0.055 rows=14 loops=1)
Index Cond: (a = '118.145.5.20'::inet)
-> Index Scan using dns_cname_cname_idx on dns_cname c1 (cost=0.56..19.70 rows=329 width=45) (actual time=0.137..0.148 rows=13 loops=14)
Index Cond: ((cname)::text = (a.name)::text)
-> Index Scan using dns_cname_cname_idx on dns_cname c2 (cost=0.56..6.33 rows=329 width=45) (actual time=0.057..0.057 rows=0 loops=189)
Index Cond: ((cname)::text = (c1.name)::text)
Planning time: 0.452 ms
Execution time: 13.012 ms
(10 rows)
Time: 13.787 ms
因此,性能差异约为100倍,这正是让我担忧的事情。 我喜欢递归CTE的便利性,并且更喜欢使用它而不是在应用程序中进行“脏”操作,但我不明白为什么
Index Scan using dns_cname_cname_idx on dns_cname c (cost=0.56..69958.06 rows=2268434 width=45) (actual time=4.732..688.456 rows=2219973 loops=2)
的成本如此之高。
我是否在CTE方面错过了重要的东西,或者问题出在其他方面?
谢谢!
更新: 我的一个朋友指出我错过了受影响行数 Index Scan using dns_cname_cname_idx on dns_cname c (cost=0.56..69958.06 rows=2268434 width=45) (actual time=4.732..688.456 rows=2219973 loops=2)
,这等于表中的总行数,如果我理解正确,它执行带有条件的完全索引扫描,我不知道漏掉了哪个条件。
结果: 应用SET LOCAL enable_mergejoin TO false;
后,执行时间大大缩短。
EXPLAIN ANALYZE WITH RECURSIVE names_traverse AS (
(
SELECT name::varchar(256), NULL::varchar(256) as cname, a FROM dns_a WHERE a = '118.145.5.20'
)
UNION ALL
SELECT c.name, c.cname, NULL::inet as a FROM names_traverse nt, dns_cname c WHERE c.cname=nt.name
)
SELECT * FROM names_traverse;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------
CTE Scan on names_traverse (cost=4746432.42..6527720.02 rows=89064380 width=1064) (actual time=0.718..45.656 rows=199 loops=1)
CTE names_traverse
-> Recursive Union (cost=0.57..4746432.42 rows=89064380 width=45) (actual time=0.717..45.597 rows=199 loops=1)
-> Index Scan using dns_a_a_idx on dns_a (cost=0.57..74.82 rows=2700 width=24) (actual time=0.716..0.717 rows=14 loops=1)
Index Cond: (a = '118.145.5.20'::inet)
-> Nested Loop (cost=0.56..296507.00 rows=8906168 width=45) (actual time=11.276..22.418 rows=92 loops=2)
-> WorkTable Scan on names_traverse nt (cost=0.00..540.00 rows=27000 width=516) (actual time=0.000..0.013 rows=100 loops=2)
-> Index Scan using dns_cname_cname_idx on dns_cname c (cost=0.56..7.66 rows=330 width=45) (actual time=0.125..0.225 rows=1 loops=199)
Index Cond: ((cname)::text = (nt.name)::text)
Planning time: 0.253 ms
Execution time: 45.697 ms
(11 rows)