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FORMAT Clause
ClickHouse supports a wide range of serialization formats that can be used on query results among other things. There are multiple ways to choose a format for SELECT output, one of them is to specify FORMAT format at the end of query to get resulting data in any specific format.
Specific format might be used either for convenience, integration with other systems or performance gain.
Default Format
If the FORMAT clause is omitted, the default format is used, which depends on both the settings and the interface used for accessing the ClickHouse server. For the HTTP interface and the command-line client in batch mode, the default format is TabSeparated. For the command-line client in interactive mode, the default format is PrettyCompact (it produces compact human-readable tables).
Implementation Details
When using the command-line client, data is always passed over the network in an internal efficient format (Native). The client independently interprets the FORMAT clause of the query and formats the data itself (thus relieving the network and the server from the extra load).