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Distributed queries

Run queries across multiple servers.

Distributed query

A distributed query pulls data from several databases with one SQL statement. In PostgreSQL a Foreign Data Wrapper (FDW) lets a remote table act like a local one.

CREATE FOREIGN TABLE remote_orders (id INT)
SERVER remote_server;

A distributed query calls a remote table.

Distributed query — details

Federation is a layout where each node keeps its own data but a central query joins them all. A JOIN can run across remote and local tables.

Practice Distributed query

Create the remote_orders foreign table from remote_server: CREATE FOREIGN TABLE remote_orders (id INT) SERVER remote_server.

Starter code

query.sql
-- distributed query
CREATE FOREIGN TABLE remote_orders

Sample solution

query.sql
CREATE FOREIGN TABLE remote_orders (id INT) SERVER remote_server;

Expected output

CREATE FOREIGN TABLE

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does a distributed query combine?

  • An index
  • One table
  • One column
  • Several databases Correct answer

Explanation: A distributed query combines several databases.

2 Create a foreign table

Question 
CREATE FOREIGN TABLE remote_orders (id INT) SERVER ____
Answer 
CREATE FOREIGN TABLE remote_orders (id INT) SERVER remote_server

Explanation: remote_server is the right answer.

3 Distributed queries: match the terms

Term Meaning
distributed distributed
foreign foreign
federation federation
remote remote

All lessons in Expert

1 Database design

  1. ER model
  2. Cardinality
  3. Surrogate keys
  4. Natural keys
  5. Denormalization
  6. Summary - Design

2 Index tuning

  1. B-tree structure
  2. Covering index
  3. Partial index
  4. Index selectivity
  5. Index maintenance
  6. Summary - Index tuning

3 Query optimizer

  1. What is an optimizer?
  2. Statistics
  3. Cost model
  4. Join strategies
  5. Plan cache
  6. Summary - Optimizer

4 Project: Performance tuning

  1. Scenario design
  2. Reading plans
  3. Adding indexes
  4. Rewriting queries
  5. Comparing results

5 Replication and sharding

  1. What is replication?
  2. Master-slave
  3. Sync vs async
  4. What is sharding?
  5. Shard key
  6. Summary - Replication and sharding

6 Distributed and big data

  1. Distributed queries
  2. MPP
  3. Columnar storage
  4. SQL on Hadoop
  5. SQL vs NoSQL
  6. Summary - Distributed and big data

7 Security and backup

  1. SQL injection
  2. Parameterised queries
  3. Authorisation
  4. Backup
  5. Recovery
  6. Summary - Security and backup

8 Expert final

  1. Design challenge
  2. Optimization challenge
  3. Security challenge
  4. Backup challenge