Sequences and other volatiles in Neki

This post details how Neki, PlanetScale's sharded Postgres, implements sequences to match native Postgres behavior. It focuses on handling volatile expressions and cross-shard synchronous evaluation. The router-only evaluation of `now()` is explained, contrasting it with `stable` expressions like `to_char()` which depend on GUCs. Sequences are classified as `volatile` and `router-only` due to the need for centralized governance to avoid duplicate values across shards. The post also defines `table expressions` as the basis for evaluating expressions per row and explains how Neki determines cardinality for correct evaluation of volatile expressions.