Add more description about SHARD_ROW_ID_BITS #21467
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Summary of Changes
Hello @qiancai, 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!
My aim with this pull request was to add more descriptive information regarding the SHARD_ROW_ID_BITS setting, specifically a notice about its value. However, the current patch only includes the addition of a new file, temp.md, containing placeholder text. This seems to be a discrepancy between the stated intent and the actual changes provided in the patch.
Highlights
- File Addition: Added a new file named temp.md containing the text 'This is a test file.'.
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Code Review
This pull request is intended to add more description about SHARD_ROW_ID_BITS, but the only change is the addition of a file named temp.md with the content "This is a test file.". This file seems unrelated to the PR's stated purpose and may have been included by mistake. Please remove this file and add the intended documentation changes.
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Code Review
This pull request adds a detailed explanation of how SHARD_ROW_ID_BITS works and how to select a value for it. The new content is valuable. I've provided a few suggestions to improve clarity, conciseness, and technical accuracy in the new explanations, following the documentation style guide.
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| > Selection of shard bits (`S`): | ||
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| > - Because the total bits of `_tidb_rowid` is 64, the number of shard bits affects the number of auto-increment bits: when the number of shard bits increases, the number of auto-increment bits decreases, and vice versa. Therefore, you need to balance the randomness of the auto-increment values and the available auto-increment space. |
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This sentence contains a minor technical inaccuracy and could be clearer.1 The term 'randomness of the auto-increment values' is misleading, as the auto-increment part is sequential. The randomness comes from the shard bits, which helps scatter the overall _tidb_rowid. Also, the trade-off happens within the 63 value bits, not the full 64 bits of the integer.2
Style Guide References
| > - Because the total bits of `_tidb_rowid` is 64, the number of shard bits affects the number of auto-increment bits: when the number of shard bits increases, the number of auto-increment bits decreases, and vice versa. Therefore, you need to balance the randomness of the auto-increment values and the available auto-increment space. | |
| Because the `_tidb_rowid` value is composed of 63 bits (excluding the sign bit), the number of shard bits affects the number of auto-increment bits. When you increase the number of shard bits, the number of auto-increment bits decreases, and vice versa. Therefore, you need to balance the scattering of row IDs with the available auto-increment space. |
Footnotes
| |--------|--------|--------------| | ||
| | 1 bit | `S` bits | `63-S` bits | | ||
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| - The values of the auto-increment bits are stored in TiKV and allocated sequentially. Each time a value is allocated, the next value is incremented by 1. The auto-increment bits ensure that the column values of `_tidb_rowid` are unique globally. When the value of the auto-increment bits is exhausted (that is, when the maximum value is reached), subsequent automatic allocations fail with the error `Failed to read auto-increment value from storage engine`. |
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This sentence is a bit long and can be rephrased for better readability and conciseness.1
Style Guide References
| - The values of the auto-increment bits are stored in TiKV and allocated sequentially. Each time a value is allocated, the next value is incremented by 1. The auto-increment bits ensure that the column values of `_tidb_rowid` are unique globally. When the value of the auto-increment bits is exhausted (that is, when the maximum value is reached), subsequent automatic allocations fail with the error `Failed to read auto-increment value from storage engine`. | |
| The auto-increment bit values are stored in TiKV and allocated sequentially, ensuring global uniqueness for `_tidb_rowid` values. If these bits are exhausted (the maximum value is reached), subsequent allocations fail with the error `Failed to read auto-increment value from storage engine`. |
Footnotes
| | 1 bit | `S` bits | `63-S` bits | | ||
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| - The values of the auto-increment bits are stored in TiKV and allocated sequentially. Each time a value is allocated, the next value is incremented by 1. The auto-increment bits ensure that the column values of `_tidb_rowid` are unique globally. When the value of the auto-increment bits is exhausted (that is, when the maximum value is reached), subsequent automatic allocations fail with the error `Failed to read auto-increment value from storage engine`. | ||
| - The value range of `_tidb_rowid`: the maximum number of bits for the final generated value = shard bits + auto-increment bits, so the maximum value is `(2^63)-1`. |
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The phrasing of this sentence can be improved for clarity.1 The part the maximum number of bits for the final generated value = shard bits + auto-increment bits is a bit awkward as it's not a real equation. It's better to state the facts more directly.
Style Guide References
| - The value range of `_tidb_rowid`: the maximum number of bits for the final generated value = shard bits + auto-increment bits, so the maximum value is `(2^63)-1`. | |
| The value of `_tidb_rowid` is determined by the shard bits and the auto-increment bits. The total number of bits for the value is 63, so the maximum value is `(2^63)-1`. |
Footnotes
What is changed, added or deleted? (Required)
Add notice about setting the value of SHARD_ROW_ID_BITS
Which TiDB version(s) do your changes apply to? (Required)
Tips for choosing the affected version(s):
By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.
For details, see tips for choosing the affected versions (in Chinese).
What is the related PR or file link(s)?
Do your changes match any of the following descriptions?