A reference table is a table that is fully replicated across all shards instead of being divided into separate partitions. This allows every shard to have a local copy of the table, reducing network overhead and ensuring fast access to frequently used data.

Reference tables are useful when you need:

  • Consistent access to shared data across all shards.
  • Fast lookups without cross-shard queries.
  • Integrity constraints on small tables used across different parts of an application.

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Examples

Some typical use cases for reference tables include:

  • eCommerce: Sales tax rates apply to all stores.
  • Common codes: A table with country codes and region names.
  • Roles: A system-wide list of roles and permissions.
  • Finance: Exchange rates.
  • Global settings: Settings or feature flags.
  • SaaS: A table defining different subscription tiers, their pricing, and feature sets.

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How to use it?

To create a reference table, use the SPQR administrative console:

\c spqr-console
CREATE REFERENCE TABLE tax_rates;
         attach table          
-------------------------------
 relation name   -> tax_rates
 distribution id -> REPLICATED
(2 rows)

Then, create the table and insert data:

\c ordersdb
CREATE TABLE tax_rates (
    country_code VARCHAR(2),
    state_code VARCHAR(2),
    tax_rate NUMERIC(5, 2),
    tax_category VARCHAR(50)
);
NOTICE: send query to shard(s) : sh1,sh2,sh3,sh4
COPY tax_rates FROM 'test_tax_rates.csv' DELIMITER '\t';
NOTICE: send query to shard(s) : sh1,sh2,sh3,sh4

Verify replication:

SELECT * FROM tax_rates /* __spqr__execute_on: sh1 */;
NOTICE: send query to shard(s) : sh1
 country_code | state_code | tax_rate | tax_category
--------------+------------+----------+--------------
 US           | CA         |   7.25   | Sales
 US           | NY         |   8.875  | Sales
 GB           |            |   20.00  | VAT
 CA           |            |   5.00   | GST
 AU           |            |   10.00  | GST
 IN           |            |   18.00  | GST
(6 rows)

SELECT * FROM tax_rates /* __spqr__execute_on: sh2 */;
NOTICE: send query to shard(s) : sh2
 country_code | state_code | tax_rate | tax_category
--------------+------------+----------+--------------
 US           | CA         |   7.25   | Sales
 US           | NY         |   8.875  | Sales
 GB           |            |   20.00  | VAT
 CA           |            |   5.00   | GST
 AU           |            |   10.00  | GST
 IN           |            |   18.00  | GST
(6 rows)

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