> ## Documentation Index
> Fetch the complete documentation index at: https://docs.pg-sharding.tech/llms.txt
> Use this file to discover all available pages before exploring further.

# Sharding

The router knows that some tables have been [split into shards](/sharding/sharded_tables). If possible, the router tries to determine on the first transaction statement to which shard this transaction should be sent.

SPQR supports both single-column and [composite (multi-column) sharding keys](/sharding/composite_keys).

## Query Routing

When a query arrives, the router extracts the sharding key from the query and determines which key range (and thus which shard) should handle it.

```sql theme={null}
-- This query works with properly configured sharding rules
-- The router extracts id=10 and routes to the appropriate shard
INSERT INTO test(id, age) VALUES (10, 16);
```

### Explicit Sharding Key

You can explicitly specify a sharding key in a SQL comment when the router cannot determine it automatically:

```sql theme={null}
-- Override automatic routing by specifying the sharding key
INSERT INTO test(id, age) VALUES (10, 16) /*__spqr__sharding_key: 30*/;
```

For composite sharding keys, specify all key values separated by commas:

```sql theme={null}
-- Explicit composite sharding key
INSERT INTO users(tenant_id, user_id, name) VALUES (1, 100, 'Alice') /*__spqr__sharding_key: 1, 100*/;
```

### Explicit Shard Selection

You can also force a query to execute on a specific shard:

```sql theme={null}
-- Execute on a specific shard regardless of key
SELECT * FROM test /*__spqr__execute_on: sh1*/;
```