provide fn to delete a database: mdb_drop(1)

[mikedilger]

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You can clear a database but not remove it. This isn't high priority, but I like things tidy.

[Kerollmops]

Hey @mikedilger 👋

I am moving this one to a future milestone to release the v0.20 soon. For your information, the official LMDB documentation states that it is not useful to close DBIs.

Close a database handle. Normally unnecessary. [..] Closing a database handle is not necessary, but lets #mdb_dbi_open() reuse the handle value. Usually it's better to set a bigger #mdb_env_set_maxdbs(), unless that value would be large.