v8.5.5: Add index lookup push down content (#22196)

[ti-chi-bot]

This is an automated cherry-pick of #22196

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• This PR is translated from: Add index lookup push down content docs-cn#20950
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[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, 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!

This pull request enhances TiDB's query optimization capabilities by introducing new optimizer hints and a system variable related to IndexLookUp pushdown. These additions provide users with finer control over how IndexLookUp operations are executed, allowing for potential performance improvements by pushing these operations closer to the data storage layer (TiKV). The changes primarily involve updating documentation to reflect these new features.

Highlights

• New Optimizer Hint: INDEX_LOOKUP_PUSHDOWN: Introduced the INDEX_LOOKUP_PUSHDOWN hint, allowing users to explicitly instruct the optimizer to push down the IndexLookUp operator to TiKV for execution, along with its usage, example execution plan, and a comprehensive list of current limitations.
• New Optimizer Hint: NO_INDEX_LOOKUP_PUSHDOWN: Added the NO_INDEX_LOOKUP_PUSHDOWN hint, which provides a mechanism to explicitly disable IndexLookUp pushdown for a specified table, especially when the tidb_index_lookup_pushdown_policy system variable is set to force or affinity-force. This hint takes precedence over INDEX_LOOKUP_PUSHDOWN.
• New System Variable: tidb_index_lookup_pushdown_policy: Documented the new tidb_index_lookup_pushdown_policy system variable, which controls whether and when TiDB pushes the IndexLookUp operator down to TiKV. It offers three value options: hint-only (default), affinity-force, and force.

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[gemini-code-assist]

Code Review

This pull request adds documentation for the new Index Lookup Pushdown feature, including the INDEX_LOOKUP_PUSHDOWN and NO_INDEX_LOOKUP_PUSHDOWN hints, and the tidb_index_lookup_pushdown_policy system variable. The changes are clear and well-structured. I have a couple of minor suggestions to improve readability and provide more context for readers.

[ti-chi-bot]

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