feat&docs: 新增关系型数据库的任务切分,以及更新各关系型数据切分键的文档。

[codecov-commenter]

Codecov Report

Base: 94.13% // Head: 94.44% // Increases project coverage by +0.31% :tada:

Coverage data is based on head (6c7016c) compared to base (5d86155). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #9 +/- ## ========================================== + Coverage 94.13% 94.44% +0.31% ========================================== Files 120 121 +1 Lines 5150 5438 +288 ========================================== + Hits 4848 5136 +288 Misses 181 181 Partials 121 121 ``` | [Impacted Files](https://codecov.io/gh/Breeze0806/go-etl/pull/9?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [storage/database/field.go](https://codecov.io/gh/Breeze0806/go-etl/pull/9?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3RvcmFnZS9kYXRhYmFzZS9maWVsZC5nbw==) | `100.00% <ø> (ø)` | | | [datax/plugin/reader/rdbm/config.go](https://codecov.io/gh/Breeze0806/go-etl/pull/9?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZGF0YXgvcGx1Z2luL3JlYWRlci9yZGJtL2NvbmZpZy5nbw==) | `100.00% <100.00%> (ø)` | | | [datax/plugin/reader/rdbm/db\_handler.go](https://codecov.io/gh/Breeze0806/go-etl/pull/9?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZGF0YXgvcGx1Z2luL3JlYWRlci9yZGJtL2RiX2hhbmRsZXIuZ28=) | `100.00% <100.00%> (ø)` | | | [datax/plugin/reader/rdbm/job.go](https://codecov.io/gh/Breeze0806/go-etl/pull/9?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZGF0YXgvcGx1Z2luL3JlYWRlci9yZGJtL2pvYi5nbw==) | `100.00% <100.00%> (ø)` | | | [datax/plugin/reader/rdbm/parameter.go](https://codecov.io/gh/Breeze0806/go-etl/pull/9?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZGF0YXgvcGx1Z2luL3JlYWRlci9yZGJtL3BhcmFtZXRlci5nbw==) | `100.00% <100.00%> (ø)` | | | [datax/plugin/reader/rdbm/split.go](https://codecov.io/gh/Breeze0806/go-etl/pull/9?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZGF0YXgvcGx1Z2luL3JlYWRlci9yZGJtL3NwbGl0Lmdv) | `100.00% <100.00%> (ø)` | | | [element/time.go](https://codecov.io/gh/Breeze0806/go-etl/pull/9?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZWxlbWVudC90aW1lLmdv) | `100.00% <100.00%> (ø)` | | | [storage/database/mysql/field.go](https://codecov.io/gh/Breeze0806/go-etl/pull/9?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3RvcmFnZS9kYXRhYmFzZS9teXNxbC9maWVsZC5nbw==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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