URL Table Engine

Queries data to/from a remote HTTP/HTTPS server. This engine is similar to the File engine.

Syntax: URL(URL, Format)

Usage

The format must be one that ClickHouse can use in
SELECT queries and, if necessary, in INSERTs. For the full list of supported formats, see
Formats.

The URL must conform to the structure of a Uniform Resource Locator. The specified URL must point to a server
that uses HTTP or HTTPS. This does not require any
additional headers for getting a response from the server.

INSERT and SELECT queries are transformed to POST and GET requests,
respectively. For processing POST requests, the remote server must support
Chunked transfer encoding.

You can limit the maximum number of HTTP GET redirect hops using the max_http_get_redirects setting.

Example

1. Create a url_engine_table table on the server :

  1. CREATE TABLE url_engine_table (word String, value UInt64)
  2. ENGINE=URL('http://127.0.0.1:12345/', CSV)

2. Create a basic HTTP server using the standard Python 3 tools and
start it:

  1. from http.server import BaseHTTPRequestHandler, HTTPServer
  3. class CSVHTTPServer(BaseHTTPRequestHandler):
  4.     def do_GET(self):
  5.         self.send_response(200)
  6.         self.send_header('Content-type', 'text/csv')
  7.         self.end_headers()
  9.         self.wfile.write(bytes('Hello,1\nWorld,2\n', "utf-8"))
  11. if __name__ == "__main__":
  12.     server_address = ('127.0.0.1', 12345)
  13.     HTTPServer(server_address, CSVHTTPServer).serve_forever()
  1. $ python3 server.py

3. Request data:

  1. SELECT * FROM url_engine_table
  1. ┌─word──┬─value─┐
  2. │ Hello │     1 │
  3. │ World │     2 │
  4. └───────┴───────┘

Details of Implementation

  • Reads and writes can be parallel
  • Not supported:
    • ALTER and SELECT...SAMPLE operations.
    • Indexes.
    • Replication.

Original article