SPM: add binding usage

[hawkingrei]

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[gemini-code-assist[bot]]

Summary of Changes

Hello @hawkingrei, I'm Gemini Code Assist[^1]! 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!

This pull request enhances the SQL Plan Management documentation by introducing a dedicated section on 'Binding Usage'. This addition aims to provide users with comprehensive information on how to track and manage the usage of SQL bindings, thereby facilitating better optimization and maintenance of query performance through the identification and handling of unused bindings.

Highlights

• New Documentation Section: A new section titled 'Binding Usage' has been added to the sql-plan-management.md document.
• Binding Usage Explanation: The new section explains how 'Binding Usage' helps collect statistics for SQL bindings, aiding in the identification and management of unused bindings to optimize query performance.
• System Variable: The tidb_enable_binding_usage system variable is introduced, which controls the collection of usage statistics for SQL plan bindings.
• Usage Query Example: An example SQL query is provided to demonstrate how to retrieve binding usage statistics, specifically the sql_digest and last_used_date from the mysql.bind_info table.

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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[Oreoxmt]

/cc @Oreoxmt

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2025-11-03 07:34:38.694526506 +0000 UTC m=+82728.137556385: :ballot_box_with_check: agreed by Oreoxmt.