Omnisharded tables

Omnisharded tables are tables that contain the same data on all shards. This is useful for storing relatively static metadata used in joins or data that doesn't fit the sharding schema of the database, e.g., list of countries, global settings, list of blocked IPs, etc.

Other names for these tables include mirrored tables and replicated tables.

Configuration

Omnisharded tables are configured in pgdog.toml:

[[omnisharded_tables]]
database = "prod"
tables = [
    "settings",
    "cities",
    "terms_of_service",
    "ip_blocks",
]

Query routing

Omnisharded tables are treated differently by the query router. Write queries are sent to all shards concurrently, while read queries are distributed evenly between shards using round robin.

If the query contains a sharding key, it will be used instead, and omnisharded tables in that query will be ignored.

Consistency

Writing data to omnisharded tables is atomic if you enabled two-phase commit.

If you can't or choose not to use 2pc, make sure writes to omnisharded tables can be repeated in case of failure. This can be achieved by using unique indexes and INSERT ... ON CONFLICT ... DO UPDATE queries.

Since reads from omnisharded tables are routed to individual shards, while two-phase commit takes place, queries to these tables may return different results for a brief period of time.