Use function instead of closures

[Leigang Zhang]

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The code of these closures is too long, resulting in a large amount of code for this function, and these closures only capture the variable db.

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Rewrite these closures with functions.

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[Gökhan Özeloğlu]

Hi @zll600. You might be right, closures can be converted to functions. What does @roseduan think about it?

[roseduan]

Thanks! I am refactoring the project, and I will consider your opinion.