ddl: Fix placement policy in alter table partition by

[mjonss]

What problem does this PR solve?

Issue Number: close #48630

Problem Summary:
When specifying PLACEMENT POLICY per partitions in ALTER TABLE t PARTITION BY ... it failed, due to missed lookup of Name -> ID of the placement policy.
Workaround is to not assign placement policy and then alter the placement policy by:
https://docs.pingcap.com/tidb/dev/placement-rules-in-sql#specify-a-placement-policy-for-a-partitioned-table

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[tiprow]

Hi @mjonss. Thanks for your PR.

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[codecov]

Codecov Report

Merging #48631 (bddbcec) into master (625dc4e) will increase coverage by 2.7953%.
Report is 54 commits behind head on master.
The diff coverage is 0.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #48631        +/-   ##
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+ Coverage   71.4230%   74.2184%   +2.7953%     
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  Files          1404       1436        +32     
  Lines        407220     428310     +21090     
================================================
+ Hits         290849     317885     +27036     
+ Misses        96417      91601      -4816     
+ Partials      19954      18824      -1130     

Flag	Coverage Δ
integration	45.5125% <0.0000%> (?)
unit	72.5788% <0.0000%> (+1.1557%)	⬆️

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

Components	Coverage Δ
dumpling	54.1220% <ø> (+0.1345%)	⬆️
parser	∅ <ø> (∅)
br	48.9444% <ø> (-4.1373%)	⬇️

[mjonss]

/retest

[tiprow]

@mjonss: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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/retest

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[mjonss]

/ok-to-test

[YangKeao]

LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-17 08:26:39.952219044 +0000 UTC m=+4410397.539329174: ☑️ agreed by lcwangchao.
• 2023-11-17 08:41:54.506607536 +0000 UTC m=+4411312.093717666: ☑️ agreed by YangKeao.

[YangKeao]

@mjonss Please triage the issue 🍻

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lcwangchao, YangKeao, zimulala

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #48750.