Pushdown simplest sorting for find command

[noisersup]

Description

Closes #2438.

The pushdown for the find command. The aggregation will come soon in other PR, just need to clarify some concerns.

The PR also changes explain command to use internal data to specify if pushdown occured, rather than using Index cond/filter fields from postgres.

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #2506 (bc504ad) into main (203467c) will increase coverage by 0.01%.
The diff coverage is 73.94%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2506      +/-   ##
==========================================
+ Coverage   64.34%   64.35%   +0.01%     
==========================================
  Files         403      403              
  Lines       19956    20029      +73     
==========================================
+ Hits        12840    12890      +50     
- Misses       6187     6204      +17     
- Partials      929      935       +6     

Impacted Files	Coverage Δ
internal/handlers/pg/pg.go	79.10% <ø> (ø)
internal/handlers/pg/pgdb/query.go	79.53% <69.51%> (-1.99%)	⬇️
internal/handlers/pg/msg_explain.go	67.96% <75.00%> (-0.79%)	⬇️
internal/handlers/pg/msg_find.go	71.96% <83.33%> (-1.26%)	⬇️
internal/handlers/common/sort.go	84.21% <100.00%> (ø)
internal/handlers/pg/msg_aggregate.go	83.18% <100.00%> (ø)
internal/handlers/pg/msg_delete.go	68.33% <100.00%> (ø)
internal/handlers/pg/pgdb/collections.go	67.85% <100.00%> (ø)
internal/handlers/pg/pgdb/database_metadata.go	82.50% <100.00%> (ø)
internal/handlers/registry/registry.go	70.96% <100.00%> (+0.96%)	⬆️

... and 1 file with indirect coverage changes

Flag	Coverage Δ
integration	57.17% <47.89%> (-0.12%)	⬇️
mongodb	5.05% <0.00%> (-0.02%)	⬇️
pg	57.10% <47.89%> (-0.12%)	⬇️
unit	25.58% <45.37%> (+0.17%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[mergify]

@noisersup this pull request has merge conflicts.

[w84thesun]

LGTM!

[rumyantseva]

The idea and implementation looks good to me. To unblock you for aggregation implementation, I approve this PR.

To add in the second PR (apart from aggregation):

• a (unit) test for iter, queryRes, err := QueryDocuments(ctxGet, tx, qp) that queryRes is returned correctly
• check code cov if branches for sort pushdown are covered

[rumyantseva]

(Please fix the failing tests)

[noisersup]

It seems that integration is not panicing right now, I've also added unit tests.

[rumyantseva]

Tested locally, looks good to me.
One tiny suggestion - as it costs nothing to include the info about sort pushdown to explain, let's add it.

[rumyantseva]

Great!

I tried it with and without the flag, works as expected!

test> db.foo.find().sort("hello").explain()
{
  queryPlanner: {
    Plan: {
      'Node Type': 'Sort',
      'Parallel Aware': false,
      'Startup Cost': 97.78,
      'Plan Width': 64,
      Plans: [
        {
          'Parent Relationship': 'Outer',
          'Parallel Aware': false,
          'Async Capable': false,
          Schema: 'test',
          Alias: 'foo_a9f37ed7',
          Output: [ '_jsonb', "(_jsonb -> 'hello'::text)" ],
          'Node Type': 'Seq Scan',
          'Startup Cost': 0,
          'Total Cost': 27,
          'Plan Rows': 1360,
          'Plan Width': 64,
          'Relation Name': 'foo_a9f37ed7'
        }
      ],
      'Async Capable': false,
      'Total Cost': 101.18,
      'Plan Rows': 1360,
      Output: [ '_jsonb', "((_jsonb -> 'hello'::text))" ],
      'Sort Key': [ "((foo_a9f37ed7._jsonb -> 'hello'::text))" ]
    }
  },
  explainVersion: '1',
  command: { find: 'foo', filter: {}, sort: { hello: 1 }, '$db': 'test' },
  pushdown: false,
  sortingPushdown: true,
  serverInfo: {
    host: 'ferretdb',
    version: '6.0.42',
    gitVersion: 'bc504ad73fbe30deb7096cc82d76f6ba8370cbaa',
    ferretdbVersion: 'v1.0.0-95-gbc504ad7-dirty'
  },
  ok: 1
}