Qqwy
How to efficiently order and filter query results using Mnesia?
This topic is somewhat related to the topic ‘Mnesia vs Cassandra (vs CouchDB vs ...) - your thoughts?’
I am currently running/building a chat application using Mnesia (to be more specific, I am using the EctoMnesia Ecto Adapter, but am rapidly approaching its limits, because it contains a couple of unfinished, unstable and non-existent functionalities)
When a user sees the chat screen for the first time, I want to show the 20 newest messages. And when the user scrolls backwards through history, I want to fetch the 10 messages before the oldest message’s datetime.
In other words: I want to select all messages older than a given datetime, order the results by inserted_at (in descending order), and then take only the first ten from them.
AFAIK, EctoMnesia will sort/limit in Elixir-land on the lists that are the results of the :mnesia-call, meaning that all calls will become slower (with O(n log n) time complexity) as more messages are part of the table.
I’d like to know:
- Is there a way for Mnesia to perform such a query faster?
- I know there is QLC, is it able to do so (faster than the ‘Elixir running on the output’ implementation) by using cursors or the like, or not?
- Or is Mnesia not at all able to do this, unless I alter my primary keys to be e.g. Snowflakes (IDs that are a combination of the current datetimestamp and a random number)?
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tty
QLC provides cursors for Mnesia.
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Since you want newest messages, any increasing integer count is sufficient for the primary key. Grab just the keys, sort and split then read only the recent 20.
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You could use a 2-stage table, put messages into a “recent table” and move older ones into a “long-term storage table”
jordiee
You could have another value in your stored object that is an integer based on previous inserts. Then in order to travel the documents 20 at a time you could take the last record from the previous search and use it as the news value to search greater then in a select search.
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