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graphql-engine/server/tests-py/queries/explain/limit_query.yaml
Rakesh Emmadi fa152d842d server/postgres: optimize SQL query generation with LIMITs 
>

### Description
>
This PR supersedes https://github.com/hasura/graphql-engine-mono/pull/1484. Apply `limit` to the table selection before joining relationship rows to improve query performance.

### Changelog

- [x] `CHANGELOG.md` is updated with user-facing content relevant to this PR. If no changelog is required, then add the `no-changelog-required` label.

### Affected components

- [x] Server

### Related Issues
->
Fix https://github.com/hasura/graphql-engine/issues/5745

### Solution and Design
>
Prior to this change, we apply `LIMIT` and `OFFSET` to the outer selection from sub-query which includes joins for relationships. Now, we move `LIMIT` and `OFFSET` (if present) to inner selection of base table. But, this isn't done always! If there are order by relationships' columns we apply at the outer selection. To know more, please refer to [source code note](https://github.com/hasura/graphql-engine-mono/pull/2078/files#diff-46d868ee45d3eaac667cebb34731f573c77d5c9c8097bb9ccf1115fc07f65bfdR652).

```graphql
query {
  article(limit: 2){
    id
    title
    content
    author{
      name
    }
  }
}
```
Before:
```sql
SELECT
  coalesce(json_agg("root"), '[]') AS "root"
FROM
  (
    SELECT
      row_to_json(
        (
          SELECT
            "_4_e"
          FROM
            (
              SELECT
                "_0_root.base"."id" AS "id",
                "_0_root.base"."title" AS "title",
                "_0_root.base"."content" AS "content",
                "_3_root.or.author"."author" AS "author"
            ) AS "_4_e"
        )
      ) AS "root"
    FROM
      (
        SELECT
          *
        FROM
          "public"."article"
        WHERE
          ('true')
      ) AS "_0_root.base"
      LEFT OUTER JOIN LATERAL (
        SELECT
          row_to_json(
            (
              SELECT
                "_2_e"
              FROM
                (
                  SELECT
                    "_1_root.or.author.base"."name" AS "name"
                ) AS "_2_e"
            )
          ) AS "author"
        FROM
          (
            SELECT
              *
            FROM
              "public"."author"
            WHERE
              (("_0_root.base"."author_id") = ("id"))
          ) AS "_1_root.or.author.base"
      ) AS "_3_root.or.author" ON ('true')
    LIMIT
      2
  ) AS "_5_root"
```
cost
```
Aggregate  (cost=0.73..0.74 rows=1 width=32)
  ->  Limit  (cost=0.15..0.71 rows=2 width=32)
        ->  Nested Loop Left Join  (cost=0.15..223.96 rows=810 width=32)
              ->  Seq Scan on article  (cost=0.00..18.10 rows=810 width=72)
              ->  Index Scan using author_pkey on author  (cost=0.15..0.24 rows=1 width=36)
                    Index Cond: (article.author_id = id)
                    SubPlan 1
                      ->  Result  (cost=0.00..0.01 rows=1 width=32)
              SubPlan 2
                ->  Result  (cost=0.00..0.01 rows=1 width=32)
```

After:
```sql
SELECT
  coalesce(json_agg("root"), '[]') AS "root"
FROM
  (
    SELECT
      row_to_json(
        (
          SELECT
            "_4_e"
          FROM
            (
              SELECT
                "_0_root.base"."id" AS "id",
                "_0_root.base"."title" AS "title",
                "_0_root.base"."content" AS "content",
                "_3_root.or.author"."author" AS "author"
            ) AS "_4_e"
        )
      ) AS "root"
    FROM
      (
        SELECT
          *
        FROM
          "public"."article"
        WHERE
          ('true')
        LIMIT
          2
      ) AS "_0_root.base"
      LEFT OUTER JOIN LATERAL (
        SELECT
          row_to_json(
            (
              SELECT
                "_2_e"
              FROM
                (
                  SELECT
                    "_1_root.or.author.base"."name" AS "name"
                ) AS "_2_e"
            )
          ) AS "author"
        FROM
          (
            SELECT
              *
            FROM
              "public"."author"
            WHERE
              (("_0_root.base"."author_id") = ("id"))
          ) AS "_1_root.or.author.base"
      ) AS "_3_root.or.author" ON ('true')
  ) AS "_5_root"
```
cost:
```
Aggregate  (cost=16.47..16.48 rows=1 width=32)
  ->  Nested Loop Left Join  (cost=0.15..16.44 rows=2 width=100)
        ->  Limit  (cost=0.00..0.04 rows=2 width=72)
              ->  Seq Scan on article  (cost=0.00..18.10 rows=810 width=72)
        ->  Index Scan using author_pkey on author  (cost=0.15..8.18 rows=1 width=36)
              Index Cond: (article.author_id = id)
              SubPlan 1
                ->  Result  (cost=0.00..0.01 rows=1 width=32)
  SubPlan 2
    ->  Result  (cost=0.00..0.01 rows=1 width=32)
```

https://github.com/hasura/graphql-engine-mono/pull/2078

Co-authored-by: Evie Ciobanu <1017953+eviefp@users.noreply.github.com>
GitOrigin-RevId: 47eaccdbfb3499efd2c9f733f3312ad31c77916f
2021-08-17 17:03:08 +00:00

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description: Explain query with limit
url: /v1/graphql/explain
status: 200
query:
query:
query: |
query {
article(limit: 2){
id
title
content
}
}
response:
- field: article
plan:
- Aggregate (cost=0.10..0.11 rows=1 width=32)
- " -> Limit (cost=0.00..0.04 rows=2 width=72)"
- " -> Seq Scan on article (cost=0.00..18.10 rows=810 width=72)"
- " SubPlan 1"
- " -> Result (cost=0.00..0.01 rows=1 width=32)"
sql: 'SELECT coalesce(json_agg("root" ), ''[]'' ) AS "root" FROM (SELECT row_to_json((SELECT "_1_e" FROM (SELECT "_0_root.base"."id"
AS "id", "_0_root.base"."title" AS "title", "_0_root.base"."content" AS "content" )
AS "_1_e" ) ) AS "root" FROM (SELECT * FROM "public"."article" WHERE
(''true'') LIMIT 2 ) AS "_0_root.base" ) AS "_2_root" '
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