CODEWITHSUNDEEP
node.jsdatabasekey-value-storeleveldb
Vlad
Vlad

Reputation: 8268

Find by value on leveldb

I've been playing with leveldb and it's really good at what it's designed to do--storing and getting key/value pairs based on keys.

But now I want to do something more advanced and find myself immediately stuck. Is there no way to find a record by value? The only way I can think of is to iterate through the entire database until I find an entry with the value I'm looking for. This becomes worse if I'm looking for multiple entries with the value (basically a "where" query) since I have to iterate through the entire database every time I try to do this type of query.

Am I trying to do what Leveldb isn't designed to do and should I be using another database instead? Or is there a nice way to do this?

Upvotes: 3

Views: 3311

Answers (1)

amirouche
amirouche

Reputation: 7873

You are right. Basically what you need to know about is key composition.

Second, you don't query by value itself in SQL WHERE clause, but using a boolean query like age = 42.

To answer your particlular question imagine you have a first key-value namespace in leveldb, where you store your objects where the value is serialized in json for instance:

 key                    |          value
-------------------------------------------------
 namespace |    uid     |          value
================================================
 users     |    1       | {name:"amz", age=32}
------------------------------------------------
 users     |    2       | {name:"abki", age=42}

In another namespace, you index users uid by age:

         key              | value
----------------------------------
 namespace    | age | uid | value
==================================
 users-by-uid |  32 | 1   | empty
----------------------------------
 users-by-uid |  42 | 2   | empty

Here the value is empty because, the key must be unique. What we could think as the value of the given rows would be uid column it's composed into the key to make each row's key unique.

In that second namespace, every key that starts with the (user-by-uid, 32) match records that answer the query age = 32.

Upvotes: 5

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