My impression is that that the process of filtering DB rows is sufficiently complex to need a full libc type execution environment. But taking a big step back in perspective, a famous example of filtering on processors connected to disks is Map-Reduce, aka Hadoop.
> What if the network controller could evaluate your firewall rules itself, so dropped packets wouldn't have to interrupt the main CPU?
Yes, this is a real thing: https://duckduckgo.com/?q=nic+packet+filter+offload
From what I remember, the IBM System Z mainframes also do this sort of thing and have dedicated IO processors that can decode XML on the fly for you and other fun things like that.
https://en.wikipedia.org/wiki/Channel_I/O
And you can find similar concepts in a standard PC today. The GPU is an example of offloading a workload to a specialized processor.
So you could use that as block storage for luster, hadoop, or similar and enable things like direct disk to disk copies.
Cool idea, seems unlikely to hit a reasonable price point though.
One of the areas that I have much hope for is near-data processing: since processors scale so much better, pretty much every peripheral device already has its own microcontroller. The idea behind NDP is basically to offload some data-heavy processing to the data layer. What if your disk layer could already preselect your data so the database wouldn't have to read and discard so many rows for each query? What if the network controller could evaluate your firewall rules itself, so dropped packets wouldn't have to interrupt the main CPU?