ddl_embedded_analyze.md: fix the format issue in title (#22037) #22039
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Summary of ChangesHello @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 introduces a new documentation page explaining the Highlights
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Code Review
This pull request adds a new documentation page for the ANALYZE feature embedded in DDL statements. The document is well-structured and provides good examples. I've made a few suggestions to improve clarity, readability, and correct minor grammatical issues, following the repository's style guide. These include rephrasing some sentences for better flow, splitting long sentences, and fixing a minor grammatical error.
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| ## Usage scenarios | ||
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| In scenarios where DDL operations alternately add or modify indexes, existing stable queries might suffer from estimation bias because the new index lacks statistics, causing the optimizer to choose suboptimal plans. For more information, see [Issue #57948](https://github.com/pingcap/tidb/issues/57948). |
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This sentence is a bit complex. For better clarity and to align with the style guide's preference for addressing the user directly, consider rephrasing it.
| In scenarios where DDL operations alternately add or modify indexes, existing stable queries might suffer from estimation bias because the new index lacks statistics, causing the optimizer to choose suboptimal plans. For more information, see [Issue #57948](https://github.com/pingcap/tidb/issues/57948). | |
| When you add or modify indexes, existing stable queries might suffer from estimation bias because the new index lacks statistics, causing the optimizer to choose suboptimal plans. For more information, see [Issue #57948](https://github.com/pingcap/tidb/issues/57948). |
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| When `tidb_stats_update_during_ddl` is `ON`, executing [`ADD INDEX`](/sql-statements/sql-statement-add-index.md) automatically runs an embedded `ANALYZE` operation after the Reorg phase finishes. This `ANALYZE` operation collects statistics for the newly created index before the index becomes visible to users, and then `ADD INDEX` proceeds with its remaining phases. | ||
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| Considering that `ANALYZE` can take time, TiDB sets a timeout threshold based on the execution time of the first Reorg. If `ANALYZE` times out, `ADD INDEX` stops waiting synchronously for `ANALYZE` to finish and continues the subsequent process, making the index visible earlier to users. This means the index statistics will be updated after `ANALYZE` completes asynchronously. |
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The term 'first Reorg' might be confusing as it implies there could be more than one. Using 'the Reorg phase' is more precise and clearer for the user.
| Considering that `ANALYZE` can take time, TiDB sets a timeout threshold based on the execution time of the first Reorg. If `ANALYZE` times out, `ADD INDEX` stops waiting synchronously for `ANALYZE` to finish and continues the subsequent process, making the index visible earlier to users. This means the index statistics will be updated after `ANALYZE` completes asynchronously. | |
| Considering that `ANALYZE` can take time, TiDB sets a timeout threshold based on the execution time of the Reorg phase. If `ANALYZE` times out, `ADD INDEX` stops waiting synchronously for `ANALYZE` to finish and continues the subsequent process, making the index visible earlier to users. This means the index statistics will be updated after `ANALYZE` completes asynchronously. |
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| ``` | ||
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| From the `ADD INDEX` example, when `tidb_stats_update_during_ddl` is `ON`, you can see that after the execution of the `ADD INDEX` DDL statement, the subsequent `EXPLAIN` output shows that statistics for the index `idx` have been automatically collected and loaded into memory (you can verify it by executing `SHOW STATS_HISTOGRAMS`). As a result, the optimizer can immediately use these statistics for range scans. If index creation or reorganization and `ANALYZE` take a long time, you can check the DDL job status by executing `ADMIN SHOW DDL JOBS`. When the `COMMENTS` column in the output contains `analyzing`, it means that the DDL job is collecting statistics. |
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This paragraph contains a long sentence with a parenthetical. For better readability, it's better to split it into separate sentences.
| From the `ADD INDEX` example, when `tidb_stats_update_during_ddl` is `ON`, you can see that after the execution of the `ADD INDEX` DDL statement, the subsequent `EXPLAIN` output shows that statistics for the index `idx` have been automatically collected and loaded into memory (you can verify it by executing `SHOW STATS_HISTOGRAMS`). As a result, the optimizer can immediately use these statistics for range scans. If index creation or reorganization and `ANALYZE` take a long time, you can check the DDL job status by executing `ADMIN SHOW DDL JOBS`. When the `COMMENTS` column in the output contains `analyzing`, it means that the DDL job is collecting statistics. | |
| From the `ADD INDEX` example, when `tidb_stats_update_during_ddl` is `ON`, you can see that after the `ADD INDEX` DDL statement is executed, the subsequent `EXPLAIN` output shows that statistics for the index `idx` have been automatically collected and loaded into memory. You can verify this by executing `SHOW STATS_HISTOGRAMS`. As a result, the optimizer can immediately use these statistics for range scans. If index creation or reorganization and `ANALYZE` take a long time, you can check the DDL job status by executing `ADMIN SHOW DDL JOBS`. When the `COMMENTS` column in the output contains `analyzing`, it means that the DDL job is collecting statistics. |
| When `tidb_stats_update_during_ddl` is `ON`, executing [`MODIFY COLUMN`](/sql-statements/sql-statement-modify-column.md) or [`CHANGE COLUMN`](/sql-statements/sql-statement-change-column.md) that reorganizes an index will also run an embedded `ANALYZE` operation after the Reorg phase completes. The mechanism is the same as for `ADD INDEX`: | ||
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| - Start collecting statistics before the index becomes visible. | ||
| - If `ANALYZE` times out, [`MODIFY COLUMN`](/sql-statements/sql-statement-modify-column.md) and [`CHANGE COLUMN`](/sql-statements/sql-statement-change-column.md) stops waiting synchronously for `ANALYZE` to finish and continues the subsequent process, making the index visible earlier to users. This means that the index statistics will be updated when `ANALYZE` finishes asynchronously. |
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There's a minor grammatical error here. Since the subject is plural (MODIFY COLUMN and CHANGE COLUMN), the verb should be 'stop' instead of 'stops'. I've also slightly rephrased the sentence for better flow.
| - If `ANALYZE` times out, [`MODIFY COLUMN`](/sql-statements/sql-statement-modify-column.md) and [`CHANGE COLUMN`](/sql-statements/sql-statement-change-column.md) stops waiting synchronously for `ANALYZE` to finish and continues the subsequent process, making the index visible earlier to users. This means that the index statistics will be updated when `ANALYZE` finishes asynchronously. | |
| - If `ANALYZE` times out, [`MODIFY COLUMN`](/sql-statements/sql-statement-modify-column.md) and [`CHANGE COLUMN`](/sql-statements/sql-statement-change-column.md) stop waiting synchronously for `ANALYZE` to finish and continues the subsequent process, making the index visible earlier to users. This means that the index statistics will be updated when `ANALYZE` finishes asynchronously. |
| +------------------------+---------+-----------+-----------------------+----------------------------------+ | ||
| 2 rows in set (0.005 sec) | ||
| ``` | ||
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This line contains extra whitespace and should be removed for correct formatting.
| 1 rows in set (0.001 sec) | ||
| ``` | ||
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| From the `MODIFY COLUMN` example, when `tidb_stats_update_during_ddl` is `ON`, you can see that after the execution of the `MODIFY COLUMN` DDL statement, the subsequent `EXPLAIN` output shows that statistics for the index `idx` have been automatically collected and loaded into memory (you can verify it by executing `SHOW STATS_HISTOGRAMS`). As a result, the optimizer can immediately use these statistics for range scans. If index creation or reorganization and `ANALYZE` take a long time, you can check the DDL job status by executing `ADMIN SHOW DDL JOBS`. When the `COMMENTS` column in the output contains `analyzing`, it means that the DDL job is collecting statistics. |
There was a problem hiding this comment.
This paragraph contains a long sentence with a parenthetical. For better readability, it's better to split it into separate sentences.
| From the `MODIFY COLUMN` example, when `tidb_stats_update_during_ddl` is `ON`, you can see that after the execution of the `MODIFY COLUMN` DDL statement, the subsequent `EXPLAIN` output shows that statistics for the index `idx` have been automatically collected and loaded into memory (you can verify it by executing `SHOW STATS_HISTOGRAMS`). As a result, the optimizer can immediately use these statistics for range scans. If index creation or reorganization and `ANALYZE` take a long time, you can check the DDL job status by executing `ADMIN SHOW DDL JOBS`. When the `COMMENTS` column in the output contains `analyzing`, it means that the DDL job is collecting statistics. | |
| From the `MODIFY COLUMN` example, when `tidb_stats_update_during_ddl` is `ON`, you can see that after the `MODIFY COLUMN` DDL statement is executed, the subsequent `EXPLAIN` output shows that statistics for the index `idx` have been automatically collected and loaded into memory. You can verify this by executing `SHOW STATS_HISTOGRAMS`. As a result, the optimizer can immediately use these statistics for range scans. If index creation or reorganization and `ANALYZE` take a long time, you can check the DDL job status by executing `ADMIN SHOW DDL JOBS`. When the `COMMENTS` column in the output contains `analyzing`, it means that the DDL job is collecting statistics. |
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This is an automated cherry-pick of #22037
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