External data for query processing
====================================

ClickHouse allows sending a server the data that is needed for processing a query, together with a SELECT query. This data is put in a temporary table (see the section "Temporary tables") and can be used in the query (for example, in IN operators).

For example, if you have a text file with important user identifiers, you can upload it to the server along with a query that uses filtration by this list.

If you need to run more than one query with a large volume of external data, don't use this feature. It is better to upload the data to the DB ahead of time.

External data can be uploaded using the command-line client (in non-interactive mode), or using the HTTP interface.

In the command-line client, you can specify a parameters section in the format
::
    --external --file=... [--name=...] [--format=...] [--types=...|--structure=...]

You may have multiple sections like this, for the number of tables being transmitted.

**--external** - Marks the beginning of the section.
**--file** -  Path to the file with the table dump, or ``-``, which refers to stdin
Only a single table can be retrieved from stdin.

The following parameters are optional:
**--name** - Name of the table. If omitted, ``_data`` is used.
**--format** - Data format in the file. If omitted, ``TabSeparated`` is used.

One of the following parameters is required:
**--types** - A comma-separated list of column types. For example, ``UInt64,String``. Columns will be named ``_1``, ``_2``, ...
**--structure** - Table structure, in the format ``UserID UInt64, URL String``. Defines the column names and types.

The files specified in ``file`` will be parsed by the format specified in ``format``, using the data types specified in ``types`` or ``structure``. The table will be uploaded to the server and accessible there as a temporary table with the name ``name``.

Examples:
::
  echo -ne "1\n2\n3\n" | clickhouse-client --query="SELECT count() FROM test.visits WHERE TraficSourceID IN _data" --external --file=- --types=Int8
  849897
  cat /etc/passwd | sed 's/:/\t/g' | clickhouse-client --query="SELECT shell, count() AS c FROM passwd GROUP BY shell ORDER BY c DESC" --external --file=- --name=passwd --structure='login String, unused String, uid UInt16, gid UInt16, comment String, home String, shell String'
  /bin/sh 20
  /bin/false      5
  /bin/bash       4
  /usr/sbin/nologin       1
  /bin/sync       1

When using the HTTP interface, external data is passed in the multipart/form-data format. Each table is transmitted as a separate file. The table name is taken from the file name. The 'query_string' passes the parameters 'name_format', 'name_types', and 'name_structure', where name is the name of the table that these parameters correspond to. The meaning of the parameters is the same as when using the command-line client.

Example:
::
  cat /etc/passwd | sed 's/:/\t/g' > passwd.tsv
  
  curl -F 'passwd=@passwd.tsv;' 'http://localhost:8123/?query=SELECT+shell,+count()+AS+c+FROM+passwd+GROUP+BY+shell+ORDER+BY+c+DESC&passwd_structure=login+String,+unused+String,+uid+UInt16,+gid+UInt16,+comment+String,+home+String,+shell+String'
  /bin/sh 20
  /bin/false      5
  /bin/bash       4
  /usr/sbin/nologin       1
  /bin/sync       1

For distributed query processing, the temporary tables are sent to all the remote servers.