[rantanplan]

The koreader sync server only stores one record per document. So essentially, the data that is stored for one document is literally byte-sized:

CREATE TABLE users (
  id BIGSERIAL PRIMARY KEY,
  username TEXT UNIQUE NOT NULL,
  password TEXT
);

CREATE TABLE progresses (
  id BIGSERIAL PRIMARY KEY,
  user_id BIGINT NOT NULL,
  document TEXT NOT NULL,
  progress TEXT NOT NULL,
  percentage REAL NOT NULL,
  device TEXT NOT NULL,
  device_id TEXT NOT NULL,
  timestamp BIGINT NOT NULL,
  FOREIGN KEY (user_id)
     REFERENCES users(id) ON DELETE CASCADE,
  UNIQUE (user_id, document)
);