PostgreSQL , 范围过滤 , 其他字段排序 , 索引 , offset , limit
在索引扫描中,如果两个字段扫描都是区间扫描,那么只能用到某个字段的过滤条件,另一个字段需要全扫描。
例如
create table t(id int, c1 int, c2 int); insert into t select generate_series(1,6000000), random()*10000, random()*10000; create index idx_t_1 on t(c1, c2); explain (analyze,verbose,timing,costs,buffers) select * from t where c1 between 1 and 10000 order by c2 limit 1; QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------ Limit (cost=25496.76..25496.76 rows=1 width=12) (actual time=5330.577..5330.578 rows=1 loops=1) Output: id, c1, c2 Buffers: shared hit=6021360 -> Sort (cost=25496.76..25571.75 rows=29997 width=12) (actual time=5330.576..5330.576 rows=1 loops=1) Output: id, c1, c2 Sort Key: t.c2 Sort Method: top-N heapsort Memory: 25kB Buffers: shared hit=6021360 -> Index Scan using idx_t_1 on public.t (cost=0.43..25346.77 rows=29997 width=12) (actual time=0.032..4526.864 rows=5999724 loops=1) Output: id, c1, c2 Index Cond: ((t.c1 >= 1) AND (t.c1 <= 10000)) Buffers: shared hit=6021360 Planning time: 0.100 ms Execution time: 5330.734 ms
(14 rows) explain (analyze,verbose,timing,costs,buffers) select * from t where c1 between 1 and 10000 and c2 between 100000 and 1111110 limit 1; QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------- Limit (cost=0.43..5.52 rows=1 width=12) (actual time=248.439..248.439 rows=0 loops=1) Output: id, c1, c2 Buffers: shared hit=21828 -> Index Scan using idx_t_1 on public.t (cost=0.43..763.89 rows=150 width=12) (actual time=248.437..248.437 rows=0 loops=1) Output: id, c1, c2 Index Cond: ((t.c1 >= 1) AND (t.c1 <= 10000) AND (t.c2 >= 100000) AND (t.c2 <= 1111110)) Buffers: shared hit=21828 Planning time: 0.120 ms Execution time: 248.578 ms
(9 rows)
原因是在复合索引中,每一个复合KEY是有序的,但是多级下来,每一层并不是有序的。例如:
那么前面的例子SQL2 实际上是层1用到了索引过滤,而层二则需要过滤。
例子SQL2 实际上是层1用到了索引过滤,并取出所有满足条件的行(因为用到了索引外的字段,所以不能用INDEX ONLY SCAN),再按层2排序。
索引结构可以参考:
《深入浅出PostgreSQL B-Tree索引结构》
那么如何优化这类SQL呢?
某个业务,数据包含了:
1、订单支付时间
2、订单号
3、唯一键 = 订单支付时间+订单号 (因为一个时间点,可能有多个订单)
业务需要取出大于某个支付时间,并按唯一键排序,偏移N条后,取一条。
DEMO表如下:
create table test(id int, c1 text, c2 timestamp);
写入6000万订单
insert into test select id, c1::text||id, c1 from (select generate_series(1,10000000) id, clock_timestamp()::timestamp(1) c1) t;
创建后期优化需要用到的索引
create index idx1 on test (c1);
create index idx2 on test (c2);
create index idx3 on test (c3);
查询语句如下:
select * from test where c2 >= '2018-01-22 16:13:25.4'::timestamp order by c1 limit 1 offset 50000; LOG: duration: 943.851 ms plan:
Query Text: select * from test where c2 >= '2018-01-22 16:13:25.4'::timestamp order by c1 limit 1 offset 50000;
Limit (cost=4714.82..4714.92 rows=1 width=40) (actual time=943.822..943.823 rows=1 loops=1) Output: id, c1, c2 Buffers: shared hit=141577 -> Index Scan using idx1 on public.test (cost=0.56..339427.36 rows=3600000 width=40) (actual time=928.122..939.856 rows=50001 loops=1) Output: id, c1, c2 Filter: (test.c2 >= '2018-01-22 16:13:25.4'::timestamp without time zone) Rows Removed by Filter: 6281766 Buffers: shared hit=141577 id | c1 | c2
---------+------------------------------+----------------------- 6414521 | 2018-01-22 16:13:25.46414521 | 2018-01-22 16:13:25.4
(1 row) Time: 944.485 ms
性能并不理想。耗费了1秒左右的时间。原因是满足条件1的数据量太大,需要完全取出后排序再OFFSET。
由于C1由C2+订单号组成,所以C1的最小值一定出现在C2中的最小值的区间中。那么可以这么优化
create or replace function get_test1(timestamp, int) returns test as $$
declare v1 timestamp; v2 text; res text;
begin set enable_seqscan=off; -- 从输入的C2的条件,得到最小的C2 -- 索引精确定位 select c2 into v1 from test where c2 >= $1 order by c2 limit 1; -- 在最小的C2中,求最小的C1 -- 索引精确定位 select min(c1) into v2 from test where c2 = v1; -- 大于最小的C1,排序返回 -- 索引offset定位 select t into res from test t where c1 >= v2 order by c1 limit 1 offset $2 ; return res::test;
end;
$$ language plpgsql strict;
使用以上优化,每一步的开销都是最小的。效果如下:
postgres=# \timing
postgres=# load 'auto_explain';
LOAD
postgres=# set auto_explain.log_nested_statements =on;
SET
Time: 0.165 ms
postgres=# set auto_explain.log_analyze =on;
SET
Time: 0.168 ms
postgres=# set auto_explain.log_buffers =on;
SET
Time: 0.144 ms
postgres=# set auto_explain.log_min_duration =0;
SET
Time: 0.161 ms
postgres=# set auto_explain.log_timing =on;
SET
Time: 0.147 ms
postgres=# set auto_explain.log_verbose =on;
SET
Time: 0.173 ms postgres=# select * from get_test1('2018-01-22 16:13:25.4'::timestamp, 50000);
LOG: duration: 0.045 ms plan:
Query Text: select c2 from test where c2 >= $1 order by c2 limit 1
Limit (cost=0.43..0.46 rows=1 width=8) (actual time=0.043..0.043 rows=1 loops=1) Output: c2 Buffers: shared hit=4 -> Index Only Scan using idx2 on public.test (cost=0.43..99814.16 rows=3333333 width=8) (actual time=0.041..0.041 rows=1 loops=1) Output: c2 Index Cond: (test.c2 >= $1) Heap Fetches: 1 Buffers: shared hit=4
LOG: duration: 0.024 ms plan:
Query Text: select min(c1) from test where c2 = v1
Result (cost=0.96..0.97 rows=1 width=32) (actual time=0.023..0.023 rows=1 loops=1) Output: $0 Buffers: shared hit=5 InitPlan 1 (returns $0) -> Limit (cost=0.56..0.96 rows=1 width=28) (actual time=0.020..0.020 rows=1 loops=1) Output: test.c1 Buffers: shared hit=5 -> Index Only Scan using idx3 on public.test (cost=0.56..31750.61 rows=79365 width=28) (actual time=0.020..0.020 rows=1 loops=1) Output: test.c1 Index Cond: ((test.c2 = $4) AND (test.c1 IS NOT NULL)) Heap Fetches: 1 Buffers: shared hit=5
LOG: duration: 57.454 ms plan:
Query Text: select t from test t where c1 >= v2 order by c1 limit 1 offset $2
Limit (cost=1699.64..1699.67 rows=1 width=92) (actual time=57.451..57.451 rows=1 loops=1) Output: t.*, c1 Buffers: shared hit=774 -> Index Scan using idx1 on public.test t (cost=0.56..125732.85 rows=3700012 width=92) (actual time=0.048..53.365 rows=50001 loops=1) Output: t.*, c1 Index Cond: (t.c1 >= '2018-01-22 16:13:25.46364521'::text) Buffers: shared hit=774
LOG: duration: 58.163 ms plan:
Query Text: select * from get_test1('2018-01-22 16:13:25.4'::timestamp, 50000);
Function Scan on public.get_test1 (cost=0.25..0.26 rows=1 width=44) (actual time=58.155..58.156 rows=1 loops=1) Output: id, c1, c2 Function Call: get_test1('2018-01-22 16:13:25.4'::timestamp without time zone, 50000) Buffers: shared hit=783 id | c1 | c2
---------+------------------------------+----------------------- 6414521 | 2018-01-22 16:13:25.46414521 | 2018-01-22 16:13:25.4
(1 row) Time: 58.503 ms
性能提升非常明显。
实际上业务方要的是c2的值(支付时间),因此完全没必要按UK(c2+订单号)来排序,也就是说,SQL可以改写成这样。
postgres=# select * from test where c2 >= '2018-01-22 16:13:25.4'::timestamp order by c2 limit 1 offset 50000;
LOG: duration: 14.945 ms plan:
Query Text: select * from test where c2 >= '2018-01-22 16:13:25.4'::timestamp order by c2 limit 1 offset 50000;
Limit (cost=1497.64..1497.67 rows=1 width=40) (actual time=14.932..14.933 rows=1 loops=1) Output: id, c1, c2 Buffers: shared hit=608 -> Index Scan using idx2 on public.test (cost=0.43..107799.04 rows=3600000 width=40) (actual time=0.030..10.541 rows=50001 loops=1) Output: id, c1, c2 Index Cond: (test.c2 >= '2018-01-22 16:13:25.4'::timestamp without time zone) Buffers: shared hit=608 id | c1 | c2
---------+------------------------------+----------------------- 6414521 | 2018-01-22 16:13:25.46414521 | 2018-01-22 16:13:25.4
(1 row) Time: 15.442 ms
从业务层出发,改一条SQL,达到了最佳的效果。(而且最后只需要一个索引即可。)
从另一个层面来看这个优化,多个字段都是范围的查询,复合索引并不是最好的选择,更好的选择可能是:分区表+单索引,或者分区索引。需要内核的功能加强。
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