一、现象
order by 排序加了limit后更慢了？
test=# explain analyze select userid from dba_users where username like '%aaaaaaaaaaaaaaaaaa%' order by userid ;QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------Sort (cost=2327.46..2328.96 rows=600 width=4) (actual time=109.316..109.318 rows=0 loops=1)Sort Key: useridSort Method: quicksort Memory: 25kB-> Bitmap Heap Scan on dba_users (cost=61.47..2299.78 rows=600 width=4) (actual time=109.311..109.312 rows=0 loops=1)Recheck Cond: ((username)::text ~~ '%aaaaaaaaaaaaaaaaaa%'::text)Rows Removed by Index Recheck: 40904Heap Blocks: exact=31502-> Bitmap Index Scan on dba_users_username_idx (cost=0.00..61.32 rows=600 width=0) (actual time=22.520..22.520 rows=40904 loops=1)Index Cond: ((username)::text ~~ '%aaaaaaaaaaaaaaaaaa%'::text)Planning Time: 0.149 msExecution Time: 109.350 ms
(11 rows)test=#
test=# explain analyze select userid from dba_users where username like '%aaaaaaaaaaaaaaaaaa%' order by userid limit 1 ; QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------Limit (cost=0.43..408.59 rows=1 width=4) (actual time=3558.960..3558.961 rows=0 loops=1)-> Index Scan using dba_users_pkey on dba_users (cost=0.43..244895.74 rows=600 width=4) (actual time=3558.958..3558.959 rows=0 loops=1)Filter: ((username)::text ~~ '%aaaaaaaaaaaaaaaaaa%'::text)Rows Removed by Filter: 6000000Planning Time: 0.171 msExecution Time: 3558.983 ms
(6 rows)test=#dba_users有600w条数据，username符合检索条件的0行， order by userid limit 1 耗时3558.983 ms，order by userid 耗时109.350 ms。
limit 1是匹配到第一条数据后就返回，这里却更慢了？ 看起来不符合预期。
表结构如下：
test=# \d+ dba_usersTable "public.dba_users"Column | Type | Collation | Nullable | Default | Storage | Compression | Stats target | Description
----------+-----------------------+-----------+----------+---------+----------+-------------+--------------+-------------userid | integer | | not null | | plain | | | username | character varying(64) | | | | extended | | | password | character varying(64) | | | | extended | | |
Indexes:"dba_users_pkey" PRIMARY KEY, btree (userid)"dba_users_password_idx" btree (password)"dba_users_username_idx" gin (username gin_trgm_ops)
Access method: heaptest=#通常我们可以给order by字段做运算或者类型转换来矫正优化器走实际更优的执行计划。
test=# explain analyze select userid from dba_users where username like '%aaaaaaaaaaaaaaaaaa%' order by userid + 0 limit 1;QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------Limit (cost=2302.78..2304.28 rows=1 width=4) (actual time=109.117..109.119 rows=0 loops=1)-> Sort (cost=2302.78..2304.28 rows=600 width=4) (actual time=109.116..109.117 rows=0 loops=1)Sort Key: ((userid + 0))Sort Method: quicksort Memory: 25kB-> Bitmap Heap Scan on dba_users (cost=61.47..2299.78 rows=600 width=4) (actual time=109.110..109.111 rows=0 loops=1)Recheck Cond: ((username)::text ~~ '%aaaaaaaaaaaaaaaaaa%'::text)Rows Removed by Index Recheck: 40904Heap Blocks: exact=31502-> Bitmap Index Scan on dba_users_username_idx (cost=0.00..61.32 rows=600 width=0) (actual time=20.856..20.856 rows=40904 loops=1)Index Cond: ((username)::text ~~ '%aaaaaaaaaaaaaaaaaa%'::text)Planning Time: 0.156 msExecution Time: 109.149 ms
(12 rows)test=#
或者order by字段和where字段建个组合索引？ 多列统计信息？这些都是一些规避的方法，优化器自身为什么选择了不优的计划？
二、分析
这个问题一直被诟病，不少文章分析过很多场景，统计信息不准？索引损坏？数据分布问题？我们一起深入探索其中的奥秘。
PostgreSQL的优化器是自底向上生成执行计划，当查询小于12个表，使用的是动态规划算法，在每个计划节点执行各种可能的path和检索方法，然后计算出最小代价path作为最优解，也就是最终的执行计划。
DEBUG跟踪这个过程。
limit 的cost计算是在adjust_limit_rows_costs函数里进行的，根据上一节点subpath的startup_cost和total_cost计算出limit count后对应的cost
如下subpath是T_IndexPath, startup_cost=0.4325 total_cost=244895.745
limit 1对应的startup_cost=0.4325 total_cost= startup_cost + (input_total_cost - input_startup_cost) count_rows / input_rows;
= 0.4325 + (244895.745 - 0.4325 ) * 1/600
= 408.5913541666666667
这个对应的是order by userid limit 1执行计划的cost：Limit (cost=0.43…408.59 rows=1 width=4)
在set_cheapest里比较表扫描的最优方式，循环比较pathlist的每个节点的startup_cost和total_cost，
以下最优total_cost是T_BitmapHeapPath,最优startup_cost是T_seqScan
最终set_cheapest函数里一直角逐出Limit节点最优的startup_cost是0.4325，total_cost是408.59135416666669，这个对应到了order by userid Limit 1 这个plan的total_cost：Limit (cost=0.43…408.59 rows=1 width=4)
在get_cheapest_fractional_path函数里返回best_path 也就是以上total_cost=408.59135416666669的path，即order by userid Limit 1的plan。
并以此创建执行计划，最终执行器执行。
那么从整个过程来看，我们需要关注的是整个计划最终节点的startup_cost和total_cost，优化器会选择最优total_cost的path作为best_path。
代价预估order by userid limit 1时，total_cost为：408.59（实际执行的total_time为：3558.961 ms）
预估order by userid + 0 limit 1时，total_cost为：2304.28（实际执行的total_time为：109.119 ms）
显而易见优化器选择了total_cost更小的408.59所在的path作为执行计划。
很明显代价预估有问题，我们开始就跟踪了order by userid limit 1的cost计算，计算的数值本身没有问题。
是优化器的缺陷？再把执行计划拉出来遛遛。
test=# explain analyze select userid from dba_users where username like '%aaaaaaaaaaaaaaaaaa%' order by userid limit 1 ; QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------Limit (cost=0.43..408.59 rows=1 width=4) (actual time=3558.960..3558.961 rows=0 loops=1)-> Index Scan using dba_users_pkey on dba_users (cost=0.43..244895.74 rows=600 width=4) (actual time=3558.958..3558.959 rows=0 loops=1)Filter: ((username)::text ~~ '%aaaaaaaaaaaaaaaaaa%'::text)Rows Removed by Filter: 6000000Planning Time: 0.171 msExecution Time: 3558.983 ms
(6 rows)test=#
cost我们之前已经计算过了，但是从actual time来看，代价预估偏差还是比较大的，
cost：Index Scan startup_cost=0.43 total_cost=244895.74 ， limit startup_cost=0.43 total_cost=408.59
从预估来看因为没有offset 所以startup_cost都是0.43，根据公式计算的 limit 的total_cost=408.59，比244895.74小了很多。
actual: Index Scan startup_time=3558.958 total_time=3558.959，limit startup_time=3558.960 total_time=3558.961
从实际执行来看Index startup_time=3558.958 total_time=3558.959，索引扫描startup_time启动代价3558.958ms？
startup_time可以理解为扫描到第一条数据的时间，这里可以虽然走了userid的pkey索引，但是根据username like ‘%aaaaaaaaaaaaaaaaaa%’ filter了600w行，就是说回表匹配了一遍所有行，因此耗时主要是在这里。
再看下limit 节点total_cost计算代价公式
*total_cost=startup_cost + (input_total_cost - input_startup_cost) count_rows / input_rows
总代价 = 父节点启动代价 + （总代价 - 启动代价）即父节点运行代价 * limit 行数/ 预估输出的总行数
这里在where 条件不含排序字段走排序字段索引情况下，有可能通过索引匹配到第一条符合条件的数据会比较久，就是说要考虑索引扫描的整体代价作为limit的启动代价（当前默认逻辑是通过索引很快找到第一条数据，然后输出limit n行，因此整体cost在这个场景下是偏小的），最极端的场景很可能是先扫描了整个索引并且回表去匹配数据，这个cost要预估进去。
因此这里的startup_cost 需要替换为total_cost, 计算公式可以调整为：
*total_cost=total_cost + (input_total_cost - input_startup_cost) count_rows / input_rows
三、方案
当where 条件不含order by字段走order by字段索引不进行sort的情况下，flag（limit_total_cost）会置为true，这个时候就走新的计算逻辑。
if (count_est != 0){double count_rows;if (count_est > 0) count_rows = (double) count_est;elsecount_rows = clamp_row_est(input_rows * 0.10);if (count_rows > *rows)count_rows = *rows;if (input_rows > 0){if (limit_total_cost){*total_cost = *total_cost + (input_total_cost - input_startup_cost)* count_rows / input_rows;}else*total_cost = *startup_cost +(input_total_cost - input_startup_cost)* count_rows / input_rows;}*rows = count_rows;if (*rows < 1)*rows = 1;}
}四、验证
执行计划显示order by userid limit 1和之前order by userid +0 limit 1的执行计划相同，sql耗时符合预期。
test=# explain analyze select userid from dba_users where username like '%aaaaaaaaaaaaaaaaaa%' order by userid limit 1 ;QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------Limit (cost=2302.78..2304.28 rows=1 width=4) (actual time=140.581..140.585 rows=0 loops=1)-> Sort (cost=2302.78..2304.28 rows=600 width=4) (actual time=140.576..140.579 rows=0 loops=1)Sort Key: useridSort Method: quicksort Memory: 25kB-> Bitmap Heap Scan on dba_users (cost=61.47..2299.78 rows=600 width=4) (actual time=140.534..140.536 rows=0 loops=1)Recheck Cond: ((username)::text ~~ '%aaaaaaaaaaaaaaaaaa%'::text)Rows Removed by Index Recheck: 40904Heap Blocks: exact=31502-> Bitmap Index Scan on dba_users_username_idx (cost=0.00..61.32 rows=600 width=0) (actual time=22.800..22.802 rows=40904 loops=1)Index Cond: ((username)::text ~~ '%aaaaaaaaaaaaaaaaaa%'::text)Planning Time: 1.413 msExecution Time: 141.032 ms
(12 rows)test=#再验证一个稍微复杂一点的场景。
修改前：
test=# explain analyze select userid from dba_users where username like '%aaaaaaaaaaaaaaaaaa%' and userid in (select city_id from measurement where logdate > '2023-02-01' and logdate < '2023-05-01' and name like '%Nickyoung%' order by city_id limit 5) ;QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Nested Loop (cost=2143.01..2185.00 rows=1 width=4) (actual time=8153.711..8153.717 rows=0 loops=1)-> HashAggregate (cost=2142.57..2142.62 rows=5 width=4) (actual time=8153.710..8153.713 rows=0 loops=1)Group Key: measurement.city_idBatches: 1 Memory Usage: 24kB-> Limit (cost=1.32..2142.56 rows=5 width=4) (actual time=8153.707..8153.710 rows=0 loops=1)-> Merge Append (cost=1.32..582419.24 rows=1360 width=4) (actual time=8153.705..8153.708 rows=0 loops=1)Sort Key: measurement.city_id-> Index Scan using measurement_y2023m02_city_id_idx on measurement_y2023m02 measurement_1 (cost=0.43..59989.39 rows=140 width=4) (actual time=855.005..855.005 rows=0 loops=1)Filter: ((logdate > '2023-02-01'::date) AND (logdate < '2023-05-01'::date) AND ((name)::text ~~ '%Nickyoung%'::text))Rows Removed by Filter: 1399554-> Index Scan using measurement_y2023m03_city_id_idx on measurement_y2023m03 measurement_2 (cost=0.43..265374.77 rows=620 width=4) (actual time=3747.483..3747.483 rows=0 loops=1)Filter: ((logdate > '2023-02-01'::date) AND (logdate < '2023-05-01'::date) AND ((name)::text ~~ '%Nickyoung%'::text))Rows Removed by Filter: 6197829-> Index Scan using measurement_y2023m04_city_id_idx on measurement_y2023m04 measurement_3 (cost=0.43..257037.48 rows=600 width=4) (actual time=3551.214..3551.214 rows=0 loops=1)Filter: ((logdate > '2023-02-01'::date) AND (logdate < '2023-05-01'::date) AND ((name)::text ~~ '%Nickyoung%'::text))Rows Removed by Filter: 6001729-> Memoize (cost=0.44..8.46 rows=1 width=4) (never executed)Cache Key: measurement.city_idCache Mode: logical-> Index Scan using dba_users_pkey on dba_users (cost=0.43..8.45 rows=1 width=4) (never executed)Index Cond: (userid = measurement.city_id)Filter: ((username)::text ~~ '%aaaaaaaaaaaaaaaaaa%'::text)Planning Time: 3.282 msExecution Time: 8153.877 ms
(24 rows)test=#
修改后：
test=# explain analyze select userid from dba_users where username like '%aaaaaaaaaaaaaaaaaa%' and userid in (select city_id from measurement where logdate > '2023-02-01' and logdate < '2023-05-01' and name like '%Nickyoung%' order by city_id limit 5) ;QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------Nested Loop (cost=5982.58..6024.57 rows=1 width=4) (actual time=0.129..0.131 rows=0 loops=1)-> HashAggregate (cost=5982.13..5982.18 rows=5 width=4) (actual time=0.128..0.130 rows=0 loops=1)Group Key: measurement.city_idBatches: 1 Memory Usage: 24kB-> Limit (cost=5982.11..5982.12 rows=5 width=4) (actual time=0.126..0.127 rows=0 loops=1)-> Sort (cost=5982.11..5985.51 rows=1360 width=4) (actual time=0.124..0.126 rows=0 loops=1)Sort Key: measurement.city_idSort Method: quicksort Memory: 25kB-> Append (cost=177.08..5959.52 rows=1360 width=4) (actual time=0.109..0.110 rows=0 loops=1)-> Bitmap Heap Scan on measurement_y2023m02 measurement_1 (cost=177.08..696.08 rows=140 width=4) (actual time=0.041..0.041 rows=0 loops=1)Recheck Cond: ((name)::text ~~ '%Nickyoung%'::text)Filter: ((logdate > '2023-02-01'::date) AND (logdate < '2023-05-01'::date))-> Bitmap Index Scan on measurement_y2023m02_name_idx (cost=0.00..177.05 rows=140 width=0) (actual time=0.039..0.039 rows=0 loops=1)Index Cond: ((name)::text ~~ '%Nickyoung%'::text)-> Bitmap Heap Scan on measurement_y2023m03 measurement_2 (cost=377.60..2665.41 rows=620 width=4) (actual time=0.031..0.031 rows=0 loops=1)Recheck Cond: ((name)::text ~~ '%Nickyoung%'::text)Filter: ((logdate > '2023-02-01'::date) AND (logdate < '2023-05-01'::date))-> Bitmap Index Scan on measurement_y2023m03_name_idx (cost=0.00..377.45 rows=620 width=0) (actual time=0.031..0.031 rows=0 loops=1)Index Cond: ((name)::text ~~ '%Nickyoung%'::text)-> Bitmap Heap Scan on measurement_y2023m04 measurement_3 (cost=377.50..2591.23 rows=600 width=4) (actual time=0.036..0.036 rows=0 loops=1)Recheck Cond: ((name)::text ~~ '%Nickyoung%'::text)Filter: ((logdate > '2023-02-01'::date) AND (logdate < '2023-05-01'::date))-> Bitmap Index Scan on measurement_y2023m04_name_idx (cost=0.00..377.35 rows=600 width=0) (actual time=0.036..0.036 rows=0 loops=1)Index Cond: ((name)::text ~~ '%Nickyoung%'::text)-> Memoize (cost=0.44..8.46 rows=1 width=4) (never executed)Cache Key: measurement.city_idCache Mode: logical-> Index Scan using dba_users_pkey on dba_users (cost=0.43..8.45 rows=1 width=4) (never executed)Index Cond: (userid = measurement.city_id)Filter: ((username)::text ~~ '%aaaaaaaaaaaaaaaaaa%'::text)Planning Time: 2.047 msExecution Time: 0.254 ms
(32 rows)test=#
五、小结
也算牛刀小试修正了下limit的cost预估。不过这种修改方式看起来是不优雅不专业的，内核中可能有很多特例，我们不可能只是一股脑堆if else switch case逻辑。就像Linus说的，排除特例完美覆盖所有情况才是好的代码。
针对这个case，我认为优化器目前limit节点的cost计算逻辑需要加强，计算公式可能需要更复杂的关系因子，或者使用更合理的数学表达式。鄙人不才，还需持续学习积累。
代价预估在一些特定场景下难免会有偏差，其中统计信息不准导致的场景可能会多一些，可以参考这篇了解下统计信息的原理<深入浅出统计信息内核原理(上)：Compressed Histogram>。
