UnQLite Users Forum

Ordering of keys?

append delete cleifer

I have noticed some inconsistency in the order of keys returned when iterating through the database. Particularly, I notice the ordering is one way when I first store data, then when I close and reopen the database, the ordering is different (reversed, typically).

I've noticed this with both file and in-memory databases.

Is the ordering stable or predictable? What's going on here?

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append delete #1. chm

Hi Charles,

This depends on the underlying KV storage engine used, and since the in-memory and the on-disk datastore uses a nearly identical HashTable algorithms, entries are returned (I mean here, when you use the cursor iterators interfaces) in a way similar to a LIFO stack.

The difference you see when entries are returned in a fresh new database is that they are kept in memory (The cache manager) in a way similar to a FIFO stack (unlike the on-disk LIFO and a little more complex in fact) before they are flushed to disk. That's the reason why you see difference in a newly created database.

Note that if the B+Tree KV storage were used, entries will be returned the same way (new or already created DB) using whatever sorting function were used.

Hope this helps,

append delete #2. cleifer

How can I indicate that I want to use the B+Tree?

append delete #3. cleifer

I should clarify that I tried `unqlite_config()` using the UNQLITE_CONFIG_KV_ENGINE verb and passing 'btree'.

append delete #4. chm

Yes, this the correct way to use the B+tree KV store, but this storage engine is not provided in the current public release of UnQLite.

append delete #5. Johannes

You can use the Larry c-code for btree on disk in combination with unqlite

append delete #6. flanhard

@Johannes Any link? Thanks

append delete #7. asdf

What's the Larry c-code?

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