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To prepare a PostgreSQL schema for sharding, you must propagate a stable shard key across tenant-scoped tables and use composite primary keys. This ensures data remains shard-local and avoids expensive rework later.

A shard-leading index prefixes standard indexes with the shard key to keep common queries shard-local. This routing pattern avoids scatter-gather operations by ensuring queries target a single shard instead of scanning all shards.

Yes, you can use foreign keys in a multi-tenant PostgreSQL database if they remain strictly shard-local. You must apply scoped uniqueness and shard-key propagation to ensure transaction patterns and joins do not cross shard boundaries.

You need mapping tables and app-level enforcement for database scaling when standard database constraints cannot remain shard-local. These patterns handle cross-shard relationships and unique constraints that a sharded architecture cannot enforce natively.

The main limitation of preparing an existing PostgreSQL schema for horizontal sharding is the extensive rework required if shard keys were not propagated initially. You must redesign primary keys, indexes, and co-located joins to prevent scatter-gather operations.