SKILL.md
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Concept
What it is
Keyspace
Logical database mapping to one or more shards. Analogous to a MySQL schema.
Shard
A horizontal partition of a keyspace, each backed by a separate MySQL instance.
VSchema
Configuration defining how tables map to shards, vindex (sharding) keys, and routing rules.
Vindex
Sharding function mapping column values to shards (hash, unicode_loose_xxhash, lookup).
VTGate
Stateless proxy that plans and routes queries to the correct shard(s).
Online DDL
Non-blocking schema migrations. On PlanetScale, use deploy requests for production changes.
PlanetScale specifics
- Branching: Git-like database branches for development; deploy requests for production schema changes.
- Connections: MySQL protocol, port
3306(direct) or443(serverless). SSL always required.
SQL compatibility
Vitess supports nearly all MySQL syntax — most applications work without query changes. Standard DML, DDL, joins, subqueries, CTEs (including recursive CTEs as of v21+), window functions, and common built-in functions all work as expected.
Known limitations:
- Stored procedures / triggers / events: Not supported through VTGate.
- **
LOCK TABLES/GET_LOCK**: Not supported through VTGate.
- **
SELECT ... FOR UPDATE**: Works within a single shard; cross-shard locking is not atomic.
- Cross-shard joins: Supported but expensive (scatter-gather). Filter by vindex column for single-shard routing.
- Correlated subqueries: May fail or perform poorly cross-shard. Rewrite as joins when possible.
- IDs: Use Vitess Sequences (a global counter in an unsharded keyspace) or app-generated IDs (UUIDs, snowflake) to avoid collisions on sharded tables.
- Aggregations on sharded tables:
GROUP BY/ORDER BY/LIMITmerge in VTGate memory. Large result sets can be slow.
- Foreign keys: Limited support. Prefer application-level referential integrity on sharded keyspaces.
References
Topic
Reference
Use for
VSchema
VSchema design, vindexes, sequences, sharding strategies
Schema Changes
Online DDL, managed migrations, ddl strategies, migration lifecycle
VReplication
MoveTables, Reshard, Materialize, VDiff, VStream
Architecture
VTGate, VTTablet, Topology Service, VTOrc, component interactions
Query Serving
Query routing, MySQL compatibility, cross-shard performance, EXPLAIN