PostgreSQL multipolygon 空间索引查询过滤精简优化 - IO，CPU放大优化
背景
在PostgreSQL中，目前对于空间对象的索引，采用的是GiST索引方法，空间树结构如下，每个ENTRY都是一个BOX：
如果对象是多边形，那么在索引结构中，会存储这个多边形的bound box。
那么对于非box类型，一定是会出现空间放大的。
另一方面，如果输入条件是个多边形，那么同样会将这个多边形的BOUND BOX作为输入条件，根据查询OP（操作符）到索引结构中找到这个输入BOUND BOX的branch。
这样，如果无效面积过多，就出现了索引扫描的IO放大和CPU放大。
例子如下：
优化方法是切割多边形，减少无效空间，使用union all合并结果。
《PostgreSQL 空间st_contains，st_within空间包含搜索优化 - 降IO和降CPU(bound box) (多边形GiST优化)》
对于multi polygon对象，BOUND BOX就可能很大。
比如这个图上的三个多边形组成的multi polygon，实际上bound box是很大的。
select * from tbl where st_contains(multi_polygon, pos);
以上SQL，在使用GiST搜索时，实际上会返回这个bound box包含的所有空间对象，而不是这几个小的多边形包含的空间对象。然后在通过check filter来过滤。这样就导致了IO和CPU放大。
优化方法如上所述，SPLIT空间对象，多个空间搜索结果UNION ALL得到最终结果。
例子
1、创建测试表
create table tbl (id int, pos geometry);
2、写入1000万个空间点
insert into tbl select id,
st_setsrid(
st_makepoint(
round((random()*(135.085831-73.406586)+73.406586)::numeric,6),
round((random()*(53.880950-3.408477)+3.408477)::numeric,6)
),
4326
) from generate_series(1,10000000) t(id);
3、创建空间索引
create index idx_tbl_pos on tbl using gist(pos);
4、使用多个polygon构造成一个multi polygon
select st_union(
array[
st_setsrid(st_makepolygon(ST_GeomFromText('LINESTRING(75.15 29.53,77 29,77.6 29.5, 75.15 29.53)')), 4326),
st_setsrid(st_makepolygon(ST_GeomFromText('LINESTRING(125.15 29.53,125 29,125.6 29.5, 125.15 29.53)')), 4326)
]
);
5、使用multi polygon搜索
explain (analyze,verbose,timing,costs,buffers) select * from tbl where st_contains(
st_union(
array[
st_setsrid(st_makepolygon(ST_GeomFromText('LINESTRING(75.15 29.53,77 29,77.6 29.5, 75.15 29.53)')), 4326),
st_setsrid(st_makepolygon(ST_GeomFromText('LINESTRING(125.15 29.53,125 29,125.6 29.5, 125.15 29.53)')), 4326)
]
),
pos
);
Bitmap Heap Scan on public.tbl (cost=156.65..13467.61 rows=3333 width=36) (actual time=41.020..5062.317 rows=2445 loops=1)
Output: id, pos
Recheck Cond: ('0106000020E610000002000000010300000001000000040000009A99999999C9524048E17A14AE873D4000000000004053400000000000003D4066666666666653400000000000803D409A99999999C9524048E17A14AE873D40010300000001000000040000009A99999999495F4048E17A14AE873D400000000000405F400000000000003D406666666666665F400000000000803D409A99999999495F4048E17A14AE873D40'::geometry ~ tbl.pos)
Filter: _st_contains('0106000020E610000002000000010300000001000000040000009A99999999C9524048E17A14AE873D4000000000004053400000000000003D4066666666666653400000000000803D409A99999999C9524048E17A14AE873D40010300000001000000040000009A99999999495F4048E17A14AE873D400000000000405F400000000000003D406666666666665F400000000000803D409A99999999495F4048E17A14AE873D40'::geometry, tbl.pos)
Rows Removed by Filter: 83589
Heap Blocks: exact=53874
Buffers: shared hit=823 read=53873 written=7641
-> Bitmap Index Scan on idx_tbl_pos (cost=0.00..155.82 rows=10000 width=0) (actual time=22.305..22.305 rows=86034 loops=1)
Index Cond: ('0106000020E610000002000000010300000001000000040000009A99999999C9524048E17A14AE873D4000000000004053400000000000003D4066666666666653400000000000803D409A99999999C9524048E17A14AE873D40010300000001000000040000009A99999999495F4048E17A14AE873D400000000000405F400000000000003D406666666666665F400000000000803D409A99999999495F4048E17A14AE873D40'::geometry ~ tbl.pos)
Buffers: shared hit=822
Planning time: 0.268 ms
Execution time: 5062.947 ms
(12 rows)
6、使用多个polgon搜索，使用union all合并结果
explain (analyze,verbose,timing,costs,buffers)
select * from tbl where st_contains(st_setsrid(st_makepolygon(ST_GeomFromText('LINESTRING(75.15 29.53,77 29,77.6 29.5, 75.15 29.53)')), 4326), pos)
union all
select * from tbl where st_contains(st_setsrid(st_makepolygon(ST_GeomFromText('LINESTRING(125.15 29.53,125 29,125.6 29.5, 125.15 29.53)')), 4326), pos);
Append (cost=156.65..27001.89 rows=6666 width=36) (actual time=1.594..11.143 rows=2445 loops=1)
Buffers: shared hit=5230
-> Bitmap Heap Scan on public.tbl (cost=156.65..13467.61 rows=3333 width=36) (actual time=1.594..8.473 rows=2016 loops=1)
Output: tbl.id, tbl.pos
Recheck Cond: ('0103000020E610000001000000040000009A99999999C9524048E17A14AE873D4000000000004053400000000000003D4066666666666653400000000000803D409A99999999C9524048E17A14AE873D40'::geometry ~ tbl.pos)
Filter: _st_contains('0103000020E610000001000000040000009A99999999C9524048E17A14AE873D4000000000004053400000000000003D4066666666666653400000000000803D409A99999999C9524048E17A14AE873D40'::geometry, tbl.pos)
Rows Removed by Filter: 2172
Heap Blocks: exact=4083
Buffers: shared hit=4133
-> Bitmap Index Scan on idx_tbl_pos (cost=0.00..155.82 rows=10000 width=0) (actual time=1.001..1.001 rows=4188 loops=1)
Index Cond: ('0103000020E610000001000000040000009A99999999C9524048E17A14AE873D4000000000004053400000000000003D4066666666666653400000000000803D409A99999999C9524048E17A14AE873D40'::geometry ~ tbl.pos)
Buffers: shared hit=50
-> Bitmap Heap Scan on public.tbl tbl_1 (cost=156.65..13467.61 rows=3333 width=36) (actual time=0.429..2.227 rows=429 loops=1)
Output: tbl_1.id, tbl_1.pos
Recheck Cond: ('0103000020E610000001000000040000009A99999999495F4048E17A14AE873D400000000000405F400000000000003D406666666666665F400000000000803D409A99999999495F4048E17A14AE873D40'::geometry ~ tbl_1.pos)
Filter: _st_contains('0103000020E610000001000000040000009A99999999495F4048E17A14AE873D400000000000405F400000000000003D406666666666665F400000000000803D409A99999999495F4048E17A14AE873D40'::geometry, tbl_1.pos)
Rows Removed by Filter: 655
Heap Blocks: exact=1076
Buffers: shared hit=1097
-> Bitmap Index Scan on idx_tbl_pos (cost=0.00..155.82 rows=10000 width=0) (actual time=0.295..0.295 rows=1084 loops=1)
Index Cond: ('0103000020E610000001000000040000009A99999999495F4048E17A14AE873D400000000000405F400000000000003D406666666666665F400000000000803D409A99999999495F4048E17A14AE873D40'::geometry ~ tbl_1.pos)
Buffers: shared hit=21
Planning time: 0.247 ms
Execution time: 11.432 ms
(24 rows)
7、写UDF，简化写多个UNION ALL
create or replace function q_mp(VARIADIC arr geometry[]) returns setof record as $$
declare
sql text := '';
var geometry;
begin
foreach var in array arr loop
sql := sql || format(' select * from tbl where st_contains(''%s''::geometry, pos) union all', var);
end loop;
sql := rtrim(sql, 'union all');
return query execute sql;
end;
$$ language plpgsql strict;
postgres=# explain (analyze,verbose,timing,costs,buffers) select * from q_mp
(
st_setsrid(st_makepolygon(ST_GeomFromText('LINESTRING(75.15 29.53,77 29,77.6 29.5, 75.15 29.53)')), 4326),
st_setsrid(st_makepolygon(ST_GeomFromText('LINESTRING(125.15 29.53,125 29,125.6 29.5, 125.15 29.53)')), 4326)
) as t(id int, pos geometry);
Function Scan on public.q_mp t (cost=0.25..10.25 rows=1000 width=36) (actual time=11.451..11.707 rows=2445 loops=1)
Output: id, pos
Function Call: q_mp(VARIADIC '{0103000020E610000001000000040000009A99999999C9524048E17A14AE873D4000000000004053400000000000003D4066666666666653400000000000803D409A99999999C9524048E17A14AE873D40:0103000020E610000001000000040000009A99999999495F4048E17A14AE873D400000000000405F400000000000003D406666666666665F400000000000803D409A99999999495F4048E17A14AE873D40}'::geometry[])
Buffers: shared hit=5230
Planning time: 0.095 ms
Execution time: 11.975 ms
(6 rows)
小结
使用UDF后，大大简化了SQL写法，同时性能得到了质的飞跃。
尽量的减少搜索条件，或者数据本身的无效面积，可以降低IO和CPU，大幅提升性能。
参考
《PostgreSQL 空间切割(st_split)功能扩展 - 空间对象网格化 (多边形GiST优化)》
《PostgreSQL 空间st_contains，st_within空间包含搜索优化 - 降IO和降CPU(bound box) (多边形GiST优化)》
《PostgreSQL 黑科技 - 空间聚集存储, 内窥GIN, GiST, SP-GiST索引》
将一个multigeometry对象展开成多个对象，可以参考PostGIS的文档，例如 :
http://postgis.net/docs/manual-2.4/ST_GeometryN.html