lmdb: Can I access different named databases in the same transaction?

Question

I want to write a lot of data to a lmdb data base with several named (sub) data bases. I run into the following problem:

• To write to one named data base, I need to open a transaction for this named data base.
• This implies: To write to another named data base, I need to open a different transaction.
• Two write transaction inside the same main data base cannot exist at the same time.
• This implies: I need to commit and close a transaction each time I want to switch from writing to one named data base to writing to another named data base.
• Creating and committing write transactions is a really slow operation.

I rather would like to keep one long-running write transaction for all write operations and commit it once --- when all the work is done.

Is this possible with lmdb (if yes, at which point did I err in my analysis)?

Answer 1

you err in your analysis at this point

• This implies: To write to another named data base, I need to open a different transaction.

One transaction handle can be used to open multiple subdatabases in an lmdb environment.

Note: a single transaction can open multiple databases. Ref

Answer 2

You can open as many named databases within the same write transaction as you like.

So:

• Open write transaction
• Open named databases as required and write to them
• Commit your transaction

As long as you take into account that you can only ever have one write-transaction at a time (read-only transactions are no problem), and that your other transactions will only see the result of your write-transaction once you commit, you can of course have one long-running write transaction.