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"The authorization code must expire shortly after it is issued. The OAuth 2.0 spec recommends a maximum lifetime of 10 minutes, but in practice, most services set the expiration much shorter, around 30-60 seconds."

Ref: https://www.oauth.com/oauth2-servers/authorization/the-authorization-response/

As authorization codes are short-lived and for single-use, they are implement as self encoded and is used quickly as one can.

They also cannot be stored in db neither encouraged to do that

I think you are talking about authorization code flow. As far as I can tell, authorization code expires very quickly. You should exchange it for access_token as soon as you can. 

Authorization code TTL is now 5 minutes by default. OAuth spec (RFC 6749) does not define any attribute to pass expires_in for authorization code. We will consider implementing it as an extension.