Do not merge until v8.5.4: MySQL compatibility about decimal insert through jdbc (#22007)

[ti-chi-bot]

This is an automated cherry-pick of #22007

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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 the developer guide by adding a new section on MySQL compatibility, specifically addressing how TiDB handles DECIMAL data insertions when the number of decimal places exceeds the defined scale. It clarifies the behavioral shift in TiDB v8.5.4 to match MySQL's consistent truncation logic, ensuring better compatibility for users.

Highlights

• Documentation Update: Added a new section to the developer guide detailing MySQL compatibility for DECIMAL data insertion.
• Behavioral Alignment: Documented the change in TiDB's DECIMAL truncation behavior from v8.5.4, which now aligns with MySQL's approach of automatically truncating extra decimal places regardless of their count.

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Changelog

• develop/dev-guide-sample-application-java-jdbc.md
  • Added a new section titled "MySQL compatibility" to explain TiDB's behavior when inserting DECIMAL values with excessive decimal places, comparing it to MySQL's behavior and detailing changes in TiDB v8.5.4.

Activity

• This pull request is an automated cherry-pick of Do not merge until v8.5.4: MySQL compatibility about decimal insert through jdbc #22007.
• The Contributor License Agreement has been signed.
• The changes are marked to apply to TiDB v8.5 versions.

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[gemini-code-assist]

Code Review

This pull request adds a section on MySQL compatibility regarding DECIMAL column insertions. The new content is clear and accurate, but it can be improved for conciseness and readability by reducing repetition, as per the repository's documentation style guide. I've provided a suggestion to make the text more direct and easier to understand.

[gemini-code-assist]

The explanation is clear, but it's a bit repetitive. The phrase "automatically truncates the extra digits and inserts the truncated data successfully" is used multiple times. To improve readability and conciseness, I suggest rephrasing this section to be more direct and avoid repetition, in line with the style guide's principle of avoiding unnecessary words (Rule 24).

Suggested change

	In MySQL, when you insert data into a `DECIMAL` column, if the number of decimal places exceeds the column's defined scale, MySQL automatically truncates the extra digits and inserts the truncated data successfully, regardless of how many extra decimal places there are.
	In TiDB v8.5.3 and earlier versions:
	- If the number of decimal places exceeds the defined scale but does not exceed 72, TiDB also automatically truncates the extra digits and inserts the truncated data successfully.
	- However, if the number of decimal places exceeds 72, the insertion fails and returns an error.
	Starting from TiDB v8.5.4, TiDB aligns its behavior with MySQL: regardless of how many extra decimal places there are, it automatically truncates the extra digits and inserts the truncated data successfully.
	When you insert data into a `DECIMAL` column, if the number of decimal places exceeds the column's defined scale, MySQL automatically truncates the extra digits and inserts the data successfully, regardless of the number of extra decimal places.
	TiDB's behavior in this scenario varies by version:
	- In TiDB v8.5.3 and earlier versions, TiDB also truncates the data and succeeds if the number of extra decimal places does not exceed 72. If it exceeds 72, the insertion fails and returns an error.
	- Starting from TiDB v8.5.4, TiDB's behavior is aligned with MySQL, always truncating the extra digits and inserting the data successfully.

References

1. The documentation should avoid unnecessary words and repetition to be clear and concise. ^(link)