Design a Column-Queryable KV Database

Design a distributed key-value database for sparse records, where each row has a primary key and a flexible set of named columns. The system must support: - `put(rowKey, columnName, value, timestamp)` - `get(rowKey, columnName)` - `getRow(rowKey)` - `queryByColumn(columnName, predicate)` for equality and range filters, such as all rows with `status = active` or `age between 20 and 30` Assume billions of rows, high write throughput, low-latency point reads, and moderate secondary-index queries. Explain the data model, storage engine, indexing strategy for column queries, sharding, replication, consistency model, and how you would handle schema evolution and hot partitions.