Join between tables in two different databases?

sql

cross-database-join

database-optimization

sql-performance

byAlex Kataev·Nov 28, 2024

Executing a cross-database join is quite straightforward:

-- Who needs two tables in one DB, when you can have the best of two DB worlds?
SELECT *
FROM Database1.Table1
JOIN Database2.Table2 ON Table1.Id = Table2.Fk_Id;

Ensure that you have the right privileges to access the tables from both databases. Also, check that the DBMS compatibility is appropriate for such a join operation.

Setting the stage: Cross-Database join

Highlighting Cross-Database strategies

The cross-database joins may experience performance impact due to aspects like network latency or varying optimization strategies across your databases.

-- Optimization strategy: If you can't beat the latency, join 'em 💪
CREATE INDEX idx_foreign_key 
ON Database1.Table1(foreign_key);

Consider opting for indexed foreign keys for speedy join operations or using dedicated links for efficient communication.

Decoding Metadata differences

Joining tables from different databases can run into:

Type mismatching: Ensure data types across tables are in sync.
Collation bust: To prevent unexpected outputs, match the collation settings.
Isolation disconnection: Maintain consistent transaction isolation level across your databases.

Delving Deeper: Security & Efficiency

Establishing secure and efficient cross-database querying necessitates adhering to practices such as the least privilege principle for granting permissions. Be sure to avoid plain text credentials within scripts or stored procedures.

-- Remember kids, say No to plain text credentials 👎
GRANT SELECT, UPDATE
ON Database1.Table1 
TO user@'localhost';

Consider employing connection pooling where possible to minimize overheads.

Alternatives

For complex scenarios, explore:

Database links or synonyms to ease your life.
Foreign Data Wrappers in PostgreSQL providing superpowers for cross-DBMS operations.
Employing mechanisms that store joined data, like caching or materialized views, can also help.