optimize: ddl operators don't need infoschema

[Song-Quan]

当前使用了 InfoSchema 但是其实不需要的：

• Grant/Revoke
• Create/Drop Database
• Create/Drop Table
• Show columns / Describe
• Drop/Alter User

[gemini-code-assist]

Summary of Changes

Hello @Song-Quan, 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!

此拉取请求旨在通过消除各种DDL和DCL操作以及SHOW COLUMNS/DESCRIBE语句对InfoSchema的不必要依赖，来优化其性能并降低开销。通过允许这些操作直接查询存储或使用其自身的验证机制，系统可以避免加载可能庞大的InfoSchema对象，从而实现更高效的执行。

Highlights

• 减少InfoSchema依赖: 多个DDL（如CREATE/DROP DATABASE, CREATE/DROP TABLE, ALTER/DROP USER）和DCL（GRANT/REVOKE）操作不再需要InfoSchema来执行，而是直接依赖存储查询或内部验证逻辑。
• 优化SHOW COLUMNS/DESCRIBE语句: SHOW COLUMNS和DESCRIBE语句现在已优化，可以直接从存储中获取列信息，从而在预处理阶段绕过InfoSchema查找。
• 引入新的预处理器标志: 预处理器中新增了一个inShowColumns标志，用于正确识别SHOW COLUMNS和DESCRIBE语句，使其能够跳过InfoSchema检查。
• 增强测试覆盖率: 新增了submit_handler_test.go测试文件，包含全面的测试用例，以验证isQueryNeedInfoSchema函数能够正确识别不需要InfoSchema的查询。

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[gemini-code-assist]

Code Review

本次代码审查主要关注了为优化 DDL 操作而进行的修改，这些修改旨在减少对 InfoSchema 的依赖。整体来看，PR 的目标明确，实现方式合理。主要改动包括：

1. 在 preprocess.go 中为 SHOW COLUMNS 语句添加了新的标志，以在预处理阶段跳过表名解析。
2. 在 executor/show.go 中，修改了 fetchShowColumns 函数，使其直接从底层存储查询列的元数据，而不是依赖 InfoSchema。
3. 在 server/submit_handler.go 中，扩展了 isQueryNeedInfoSchema 函数，以识别更多可以绕过加载 InfoSchema 的 DDL 语句。
4. 为 isQueryNeedInfoSchema 函数添加了全面的单元测试，确保了逻辑的正确性。

这些改动是有效的，并且有对应的测试覆盖。我提出了一些小的建议，主要涉及代码风格和错误处理，以期提高代码的可读性和健壮性。

[jingshi-ant]

针对其它需要infoSchema的场景，如果db/table过多，应该还是会很慢，会有业务影响吗？以及有改进思路吗

[Song-Quan]

会有影响，可以考虑增加 cache 或者在加载 infoschema 的时候，如果 db 过多，就选择全表扫描。
这个可以后面再提 pr