*: support building infoschema V2, and introduce @@tidb_schema_cache_size

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #51256
ref #50959

!!! Need to merge #51058 first before reviewing this PR. (DONE)

Problem Summary:

What changed and how does it work?

1. The infoschema.Builder now support building both infoschema v1 and v2

Before this change, when infoschema Builder builds the infoschema v2, it depends on building the infoschema v1 object
and rely on an infoschemaProxy object to pretender to be an InfoSchema instance.
Now it support build and use infoschemaV2 object directly.

2. Introduce the @@tidb_schema_cache_size global session variable

The variable work as a switch for us to enable v2 if it's value is not equal to 0.
We can now change to infoschema v2 and change back to v1, it make the testing easier.

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

Caution: if we expose this global variable, but infoschema v2 implementation has not finish yet, modify the default value would be buggy. There are two options:

• 1. hide it totally from the document, so nobody would know and modifty it.
• 2. add this change to the document, but mark as expiremental and notify the consequence explicitly.

I would prefer 1 because it's still at an early stage for infoschema v2.

• Contains experimental features
• Changes MySQL compatibility

Release note

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

Introduce `@@tidb_schema_cache_size` global session variable, infoschema V2 implementation would be used when its value is not zero

[tiprow]

Hi @tiancaiamao. Thanks for your PR.

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[codecov]

Codecov Report

Merging #51257 (eb277e9) into master (44b1598) will increase coverage by 2.1547%.
Report is 1 commits behind head on master.
The diff coverage is 97.9452%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #51257        +/-   ##
================================================
+ Coverage   70.6356%   72.7904%   +2.1547%     
================================================
  Files          1462       1462                
  Lines        435011     435137       +126     
================================================
+ Hits         307273     316738      +9465     
+ Misses       108424      98464      -9960     
- Partials      19314      19935       +621     

Flag	Coverage Δ
integration	49.0278% <73.2876%> (?)
unit	70.4249% <97.9452%> (+0.0055%)	⬆️

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

Components	Coverage Δ
dumpling	53.9957% <ø> (ø)
parser	∅ <ø> (∅)
br	51.4270% <ø> (+5.5851%)	⬆️

[tiancaiamao]

/retest-required

[tiprow]

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[ywqzzy]

When modify the variable, should we force trigger reload infoschema?

[tiancaiamao]

Yes, we can, but it seems we need to register callback function in SetGlobal to call the function in domain.
How to do that is a bit complex, considering infoschema reload is called eventually anyway, I skip the force trigger part.

[GMHDBJD]

LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-29 03:30:58.556020368 +0000 UTC m=+1105547.303643479: ☑️ agreed by ywqzzy.
• 2024-02-29 07:14:34.059950577 +0000 UTC m=+1118962.807573688: ☑️ agreed by GMHDBJD.

[easonn7]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: easonn7, GMHDBJD, ywqzzy

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Needs approval from an approver in each of these files:

• pkg/domain/OWNERS [GMHDBJD]
• pkg/infoschema/OWNERS [GMHDBJD,ywqzzy]
• pkg/sessionctx/variable/OWNERS [easonn7]

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