sql - 如何使用子查询优化 sql 查询，也许通过横向连接？

Question

我正在尝试优化复杂的 sql 查询，它将在每次 map 绑定(bind)框更改时执行。我认为 INNER LATERAL JOIN 会是最快的，但事实并非如此。有谁知道如何加快此查询以及如何更好地利用 LATERAL JOIN？

我做过的最快查询:

SELECT r0."id", r0."name" 
FROM "hiking"."routes" AS r0 
INNER JOIN "hiking"."hierarchy" AS h1 ON r0."id" = h1."parent" 
INNER JOIN (SELECT DISTINCT unnest(s0."rels") AS "rel" 
            FROM "hiking"."segments" AS s0 
            WHERE (ST_Intersects(s0."geom", ST_SetSrid(ST_MakeBox2D(ST_GeomFromText('POINT(1285982.015631 7217169.814674)', -1), ST_GeomFromText('POINT(2371999.313507 6454022.524275)', -1)), 3857)))) AS s2 ON TRUE 
WHERE (s2."rel" = h1."child");

Planning time: ~0.605 ms Execution time: ~37.232 ms

实际上和上面一样，但是使用LATERAL JOIN，是不是更慢了？

SELECT r0."id", r0."name" 
FROM "hiking"."routes" AS r0 
INNER JOIN "hiking"."hierarchy" AS h1 ON r0."id" = h1."parent" 
INNER JOIN LATERAL (SELECT DISTINCT unnest(s0."rels") AS "rel" 
                    FROM "hiking"."segments" AS s0 
                    WHERE (ST_Intersects(s0."geom", ST_SetSrid(ST_MakeBox2D(ST_GeomFromText('POINT(1285982.015631 7217169.814674)', -1), ST_GeomFromText('POINT(2371999.313507 6454022.524275)', -1)), 3857)))) AS s2 ON TRUE 
WHERE (s2."rel" = h1."child");

Planning time: ~1.353 ms Execution time: ~38.518 ms

子查询中包含子查询的最慢查询(这是我的第一个，所以我对其进行了一些改进):

SELECT r0."id", r0."name" 
FROM "hiking"."routes" AS r0 
INNER JOIN (SELECT DISTINCT h0."parent" AS "parent" 
            FROM "hiking"."hierarchy" AS h0 
            INNER JOIN (SELECT DISTINCT unnest(s0."rels") AS "rel" 
                        FROM "hiking"."segments" AS s0 
                        WHERE (ST_Intersects(s0."geom", ST_SetSrid(ST_MakeBox2D(ST_GeomFromText('POINT(1285982.015631 7217169.814674)', -1), ST_GeomFromText('POINT(2371999.313507 6454022.524275)', -1)), 3857)))) AS s1 ON TRUE 
            WHERE (h0."child" = s1."rel")) AS s1 ON TRUE 
WHERE (r0."top" AND (r0."id" = s1."parent"));

Planning time: ~1.017 ms Execution time: ~41.288 ms

Answer 1

如果不了解您的数据库，很难重现您的查询逻辑，但我会尝试，所以请耐心等待:

SELECT r0."id", r0."name" 
FROM "hiking"."routes" AS r0 
INNER JOIN "hiking"."hierarchy" AS h1 ON r0."id" = h1."parent" 
WHERE 
  EXISTS (
    SELECT 1
    FROM "hiking"."segments" AS s0 
    WHERE (
      ST_Intersects(
        s0."geom",
        ST_SetSrid(ST_MakeBox2D(ST_GeomFromText('POINT(1285982.015631 7217169.814674)', -1), ST_GeomFromText('POINT(2371999.313507 6454022.524275)', -1)),
        3857)))
      AND array[h1."child"] <@ s0."rels");

有两点:

1. 按EXISTS 或NOT EXISTS 过滤数据有时比加入更快
2. 您可以使用数组比较运算符，而不是取消嵌套数组字段以将其元素与某个值进行比较。拥有适当的 GIN 索引会更快(文档 here 和 here)。

这是一个简单的例子，如何在数组上使用索引以及它如何更快:

create table foo(bar int[]);
insert into foo(bar) select array[1,2,3,x] from generate_series(1,1000000) as x;
create index idx on foo using gin (bar); // Note this
select * from foo where 666 in (select unnest(bar)); // 6936,345 ms on my HW
select * from foo where array[666] <@ bar; // 45,524 ms