Add index lookup push down content #22196
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Summary of ChangesHello @Oreoxmt, 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 integrates new documentation for advanced query optimization features in TiDB. It introduces specific optimizer hints and a system variable designed to manage the pushdown of Highlights
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Code Review
This pull request adds documentation for the new INDEX_LOOKUP_PUSHDOWN feature, including optimizer hints and a related system variable. The documentation is well-written and clear. My feedback includes several minor suggestions to improve formatting and ensure consistency with the style guide, primarily by adding backticks to code-related terms and correcting list formatting.
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| When the `INDEX_LOOKUP_PUSHDOWN` hint is enabled, the outermost Build operator that originally runs on the TiDB side in the execution plan is replaced with `LocalIndexLookUp`, which is then pushed down to TiKV for execution. While scanning the index, TiKV attempts to read row data by performing local table lookups. Because the index and row data might be distributed across different Regions, the pushdown request might not cover all target rows. Therefore, TiDB retains the `TableRowIDScan` operator to fetch any rows that are not matched on the TiKV side. |
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To improve readability and maintain consistency with the style guide, please wrap the hint name and operator names in backticks.
| When the `INDEX_LOOKUP_PUSHDOWN` hint is enabled, the outermost Build operator that originally runs on the TiDB side in the execution plan is replaced with `LocalIndexLookUp`, which is then pushed down to TiKV for execution. While scanning the index, TiKV attempts to read row data by performing local table lookups. Because the index and row data might be distributed across different Regions, the pushdown request might not cover all target rows. Therefore, TiDB retains the `TableRowIDScan` operator to fetch any rows that are not matched on the TiKV side. | |
| When the `INDEX_LOOKUP_PUSHDOWN` hint is enabled, the outermost `Build` operator that originally runs on the TiDB side in the execution plan is replaced with `LocalIndexLookUp`, which is then pushed down to TiKV for execution. While scanning the index, TiKV attempts to read row data by performing local table lookups. Because the index and row data might be distributed across different Regions, the pushdown request might not cover all target rows. Therefore, TiDB retains the `TableRowIDScan` operator to fetch any rows that are not matched on the TiKV side. |
References
- Code snippets, command names, options, and paths should be in backticks. (link)
optimizer-hints.md
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| * Isolation levels other than `REPEATABLE-READ` are not supported. | ||
| - [Follower Read](/follower-read.md) is not supported. | ||
| - [Stale Read](/stale-read.md) and [reading historical data using `tidb_snapshot`](/read-historical-data.md) are not supported. | ||
| * The pushed-down `LocalIndexLookUp` operator does not support keep order. If the execution plan includes an `ORDER BY` clause on index columns, the query falls back to regular `IndexLookUp`. | ||
| * The pushed-down `LocalIndexLookUp` operator does not support sending Coprocessor requests in pagination mode. | ||
| * The pushed-down `LocalIndexLookUp` operator does not support [Coprocessor Cache](/coprocessor-cache.md). |
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I've made a few adjustments for consistency and clarity:
- Changed the bullet points from
-to*to maintain a consistent list format. - Wrapped technical terms like
tidb_snapshot,LocalIndexLookUp,ORDER BY,IndexLookUp, andCoprocessorin backticks as per the style guide.
| * Isolation levels other than `REPEATABLE-READ` are not supported. | |
| - [Follower Read](/follower-read.md) is not supported. | |
| - [Stale Read](/stale-read.md) and [reading historical data using `tidb_snapshot`](/read-historical-data.md) are not supported. | |
| * The pushed-down `LocalIndexLookUp` operator does not support keep order. If the execution plan includes an `ORDER BY` clause on index columns, the query falls back to regular `IndexLookUp`. | |
| * The pushed-down `LocalIndexLookUp` operator does not support sending Coprocessor requests in pagination mode. | |
| * The pushed-down `LocalIndexLookUp` operator does not support [Coprocessor Cache](/coprocessor-cache.md). | |
| * Isolation levels other than `REPEATABLE-READ` are not supported. | |
| * [Follower Read](/follower-read.md) is not supported. | |
| * [Stale Read](/stale-read.md) and [reading historical data using `tidb_snapshot`](/read-historical-data.md) are not supported. | |
| * The pushed-down `LocalIndexLookUp` operator does not support keep order. If the execution plan includes an `ORDER BY` clause on index columns, the query falls back to regular `IndexLookUp`. | |
| * The pushed-down `LocalIndexLookUp` operator does not support sending `Coprocessor` requests in pagination mode. | |
| * The pushed-down `LocalIndexLookUp` operator does not support [Coprocessor Cache](/coprocessor-cache.md). |
| - This variable is used to set the concurrency of the `index lookup join` algorithm. | ||
| - A value of `-1` means that the value of `tidb_executor_concurrency` will be used instead. | ||
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| ### tidb_index_lookup_pushdown_policy <span class="version-mark">New in v8.5.5 and v9.0.0</span> |
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To follow the style guide, please wrap the system variable name in the heading with backticks.
| ### tidb_index_lookup_pushdown_policy <span class="version-mark">New in v8.5.5 and v9.0.0</span> | |
| ### `tidb_index_lookup_pushdown_policy` <span class="version-mark">New in v8.5.5 and v9.0.0</span> |
References
- Code snippets, command names, options, and paths should be in backticks. (link)
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