planner: update doc for Instance plan cache

[qw4990]

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[gemini-code-assist]

Hello @qw4990, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

Summary of Changes

Gemini-code-assist here, providing a summary of this pull request. This PR focuses on updating the documentation for TiDB's prepared plan cache. The primary goal is to introduce and explain the newly supported instance-level plan cache feature, differentiating it from the existing session-level cache. The changes cover how to enable and configure the instance cache, how to monitor its contents using new system tables, how to manage its memory usage, and clarifies the current capabilities of the ADMIN FLUSH PLAN_CACHE statement regarding instance cache.

Highlights

• Introduction of Instance Plan Cache: Adds a new section explaining the concept of instance-level plan cache, which is shared across all sessions in an instance, contrasting it with the session-level cache.
• Enabling Instance Plan Cache: Documents the tidb_enable_instance_plan_cache system variable (introduced in v8.4.0) used to switch between session and instance plan caching.
• Viewing Instance Plan Cache Contents: Introduces the information_schema.tidb_plan_cache and information_schema.cluster_tidb_plan_cache system tables (available after v8.5.0) for inspecting entries in the instance-level plan cache, including a SQL example.
• Instance Plan Cache Memory Management: Adds documentation for the tidb_instance_plan_cache_max_size and tidb_instance_plan_cache_reserved_percentage system variables used to control the memory limit and purge behavior of the instance plan cache.
• Clearing Plan Cache: Adds a note under the 'Clear execution plan cache' section clarifying that the ADMIN FLUSH PLAN_CACHE statement currently only supports clearing the session-level cache.
• Platform-Specific Notes: Includes <CustomContent> blocks to add notes specific to TiDB and TiDB Cloud regarding potential plan cache misses due to memory limits.

Changelog

• sql-prepared-plan-cache.md
  • Added a new section 'Session and Instance Level Prepared Plan Cache' starting around line 20.
  • Documented the tidb_enable_instance_plan_cache system variable for enabling instance-level cache.
  • Included a SQL example demonstrating plan sharing across sessions with instance-level cache enabled.
  • Introduced and provided examples for the information_schema.tidb_plan_cache and information_schema.cluster_tidb_plan_cache system tables for viewing instance cache entries (after v8.5.0).
  • Added platform-specific notes about plan cache misses under the 'Plan Cache Memory Usage' section (around line 279).
  • Added a new subheading 'Session Level Plan Cache Memory Management' (around line 291).
  • Added a new subheading 'Instance Level Plan Cache Memory Management' (around line 316).
  • Documented the tidb_instance_plan_cache_max_size and tidb_instance_plan_cache_reserved_percentage system variables.
  • Added a note under 'Clear execution plan cache' (around line 324) about the current limitation of ADMIN FLUSH PLAN_CACHE for instance cache.

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[gemini-code-assist]

Code Review

This pull request provides a valuable update to the documentation for the Instance Plan Cache feature. The new sections on session vs. instance level caches, viewing cache entries, and memory management are well-structured and informative. The examples provided are clear and helpful for users to understand the functionality.

My review focuses on enhancing clarity, conciseness, and adherence to the TiDB documentation style guide. The suggestions aim to make the text even more user-friendly and precise. Overall, the changes significantly improve the documentation for this feature.

Summary of Findings

• Clarity and Phrasing: Several sentences were updated for better clarity, flow, and formality, aligning with the technical documentation style.
• Punctuation and Formatting: Minor punctuation (e.g., extra space before a period) and formatting of calculations were adjusted.
• Terminology Consistency: Ensured consistent use of terms like "clearing" plan cache.
• Tense Consistency: Adjusted verb tenses to prefer present tense for describing system behavior, as per the style guide.

Merge Readiness

The pull request is in good shape and the documentation updates are valuable. The suggested changes are minor and aim to further improve clarity and adherence to the style guide. As per the documentation style guide, I am providing these comments for consideration and not requesting changes to block the merge. I am not authorized to approve pull requests; please ensure other reviewers approve these changes before merging.

[qw4990]

Not a big update, please help review @songrijie @Oreoxmt

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[time-and-fate]

So we will have another doc PR when it's enabled by default and admin flush global plan_cache is supported, right?

[time-and-fate]

I think this description is different from the description in the system variables.

[songrijie]

I previously created an illustration for this. Is it better to include it?

[time-and-fate]

After this change, the tag is misplaced.
It's already wrapped in a <CustomContent platform="tidb"> tag here.

[qw4990]

So we will have another doc PR when it's enabled by default and admin flush global plan_cache is supported, right?

Yes, but it's the next version, not v9.0.

[songrijie]

We shall recommend customers to select instances plan cache, as it caches more plans with given memory size. Otherwise, customers would ask which one should be chosen.

[songrijie]

Any scenario we recommend session plan cache?

[songrijie]

The initial size of instance plan cache is small. We'd better mention setting proper cache size.

[songrijie]

Shall we include a page to describe the view under https://docs.pingcap.com/zh/tidb/stable/information-schema/ ?