When leveraging Custom Entity Store to store custom content entities, we have the possibility of declaring attributes of type "any" in the manifest.

For other attribute types we can query what we need making use of index, using partitions and ranges.

However, for attributes of type "any" there are no means to filter this attribute to retrieve specific elements of this array

As an example, below we are storing a custom entity named "user", for which we have an attribute named "customAttributes" of type any:

resolver.define('setData', async (req) => {
    await storage.entity('users').set('user12345', {
        key: 'user12345',
        email: 'user12345@gmail.com',
        mobile: '9876543210',
        customAttributes: [
            {
                fieldName: 'birth date',
                fieldValue: '28/04/2001'
            },
            {
                fieldName: 'languages known',
                fieldValue: ['english', 'tamil'],
            },
            {
                fieldName: 'skills',
                fieldValue: ['development', 'testing']
            }
        ]
    });
    return 'stored'
}); 

Some customers and partners already pointed out that this is a feature they need, specially because there's a limitation of 20 records per attribute per request at max.