planner: generate wrong plan when update has subquery (#25660)

[ti-srebot]

cherry-pick #25660 to release-5.1
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/25698

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---

Signed-off-by: Weizhen Wang wangweizhen@pingcap.com

What problem does this PR solve?

Issue Number: close #25658

Problem Summary:

What is changed and how it works?

if finding the subquery in the UPDATE's assignment list, the point_get return null.

What's Changed:

How it Works:

it change plan. the plan is shown below.

Check List

Tests

• Manual test

CREATE TABLE `users` (
  `id` bigint(20) unsigned NOT NULL,
  `name` longtext DEFAULT NULL,
  `age` bigint(20) unsigned DEFAULT NULL,
  `company_id` bigint(20) DEFAULT NULL,
  `manager_id` bigint(20) unsigned DEFAULT NULL,
  `active` tinyint(1) DEFAULT NULL,
  `created_at` datetime(3) DEFAULT NULL,
  `updated_at` datetime(3) DEFAULT NULL,
  `deleted_at` datetime(3) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_users_deleted_at` (`deleted_at`)
);
insert into users(id, company_id, name, updated_at) values(239, 15, 'Company 2 id 15', '2021-03-01 12:12:12.987');
create table companies(id bigint primary key, name longtext default null);
insert into companies values(14, 'Company 1 id 14');
insert into companies values(15, 'Company 2 id 15');
UPDATE `users` SET `company_id`=14,`name`=(SELECT `name` FROM `companies` WHERE companies.id = users.company_id),`updated_at`='2021-04-27 10:28:31.459' WHERE `id` = 239;

Release note

• No release note

[ti-srebot]

/run-all-tests

[ti-srebot]

@hawkingrei please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/tidb/invitations

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• AilinKid
• djshow832

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[zhouqiang-cl]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 4ec840e

[ti-chi-bot]

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