# Key/values of dense arrays

**URL:** https://forum.tiledb.com/t/key-values-of-dense-arrays/52
**Category:** Uncategorized
**Created:** [April 1, 2019, 7:08pm UTC](https://forum.tiledb.com/t/key-values-of-dense-arrays/52 "2019-04-01T19:08:49Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![dakoner](https://avatars.discourse-cdn.com/v4/letter/d/2acd7d/32.png) [@dakoner](https://forum.tiledb.com/u/dakoner)
#### Post date: [April 1, 2019, 7:08pm UTC](https://forum.tiledb.com/t/key-values-of-dense-arrays/52/1 "2019-04-01T19:08:49Z")

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Hi, I am learning how to model my application domain data in TileDB. For exmaple, I have many images and I can easily save those as dense arrays. But then, I want to group a collection of images together by a key.

We want to access all the images with a key; in Python, I’d model this as a dictionary with string keys and numpy array values.

Looking at the docs, I see TileDB has key-value data type- can the value itself be a dense array? My read of the documentation suggests that the values are just dumb string/byte arrays.

I see there is also an object hierarchy which appears to be a distinct/separate feature from key-value data type (is that correct?). And that feature is documented to allow nested structures.

So my guess is that I would use the object hierarchy, with multiple levels, to handle the top-level access and then reach into the actual dense arrays.

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### Author: ![stavros](https://avatars.discourse-cdn.com/v4/letter/s/51bf81/32.png) [@stavros](https://forum.tiledb.com/u/stavros)
#### Post date: [April 1, 2019, 7:20pm UTC](https://forum.tiledb.com/t/key-values-of-dense-arrays/52/2 "2019-04-01T19:20:51Z")

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Hi there,

Thanks for reaching out!

You could group your images (modeled as dense arrays) in a hierarchy of groups (i.e., folders / common array URI prefixes), instead of actually using a key-value structure. Then the common URI path prefix of the images/arrays becomes your “key”. This must be the easiest and probably fastest solution to this problem.

To retrieve the arrays associated with a group, all you need to do is list the corresponding URI prefix. Note though that this may incur some non-negligible cost for backends such as S3. If you foresee numerous such group listings, you may want to use a fast external key-value store (e.g., RocksDB?) for listing by common prefix.

Thanks for your interest in TileDB. Please let us know if you have any other questions.

Best,  
Stavros