Proposal: Data Overlay files for fast targeted writes

[wjones127]

Motivation

We want to make writing granular cell-level updates fast. For example: fill in NULL embeddings that are present in 1% of rows. Current solutions require too much write amplification.

Solution

Allow each fragment to have Overlay Files: data files that contain a subset of columns and rows, which replace values in data files.

Overview

Data.Overlay.Overview.-.1080p.mov

Spec changes

The proposal is written in this PR: #7381 Comment directly on that PR to discuss the proposal.