Re: Learned Index

On 12/12/2017 12:16 PM, Laurenz Albe wrote:
> I have read into the paper.
>
> This may be interesting or not, but the paper is very vague about
> its concepts and algorithms, so it's hard to tell.
>
> I'd say that the paper does not meet publication standards.
>
> For example, they say that their results were generated by comparing
> a B-tree implementation with "learned indexes using a 2-stage
> RMI model and different second-stage sizes (i.e., 10k, 50k, 100k, and 
> 200k)",
> but they don't say exactly what the neural network in these stages is
> (at least it is not obvious to me).  Their "Learning Index Framework" 
> (LIF)
> is described with a few vague sentences and a reference to the literature
> saying that is where they got some ideas from.
That is not the answer, but gives us the idea of which kind of neural 
networks was used: "For this paper, we only focused on 2 types of 
models, simple neural nets with zero to two fully-connected hidden 
layers and ReLU activation functions and a layer width of up to 32 
neurons and B-Trees (a.k.a. decision trees)".
> There is also no clear concept of how these indexes should handle
> data modifications, so I think that there are some loose ends to be
> tied up before it is ready for implementation.
>
> Finally, I don't see any clear statement as to the error guarantees
> that the neural network prediction can give, and if it is possible that
> it may degrade to scanning relevant parts of the table in some cases.
No guarantees are provided (I don't think it is even possible), besides 
the guarantee that if the error of the neural network prediction is more 
than the error of B-tree prediction, B-tree will be used: "Note, that 
hybrid indexes allow us to bound the worst case performance of learned 
indexes to the performance of B-Trees".

Oleg Ivanov
Postgres Professional
The Russian PostgreSQL Company