ClickHouse/docs/en/formats/capnproto.md
2018-03-30 03:15:52 +03:00

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# CapnProto
Cap'n Proto is a binary message format similar to Protocol Buffers and Thrift, but not like JSON or MessagePack.
Cap'n Proto messages are strictly typed and not self-describing, meaning they need an external schema description. The schema is applied on the fly and cached for each query.
```sql
SELECT SearchPhrase, count() AS c FROM test.hits
GROUP BY SearchPhrase FORMAT CapnProto SETTINGS schema = 'schema:Message'
```
Where `schema.capnp` looks like this:
```
struct Message {
SearchPhrase @0 :Text;
c @1 :Uint64;
}
```
Schema files are in the file that is located in the directory specified in [ format_schema_path](../operations/server_settings/settings.md#server_settings-format_schema_path) in the server configuration.
Deserialization is effective and usually doesn't increase the system load.