How to Efficiently Select Rows from One Table Not Present in Another in PostgreSQL?

Retrieving Missing IP Addresses in PostgreSQL

This guide demonstrates several methods to identify IP addresses present in the login_log table but absent from the ip_location table within a PostgreSQL database.

Method 1: NOT EXISTS

Generally considered the most efficient approach in PostgreSQL:

SELECT ip 
FROM   login_log l 
WHERE  NOT EXISTS (
   SELECT 1 -- A simple SELECT is sufficient here
   FROM   ip_location
   WHERE  ip = l.ip
   );

Method 2: LEFT JOIN/IS NULL

A concise and often efficient alternative:

SELECT l.ip 
FROM   login_log l 
LEFT   JOIN ip_location i USING (ip) -- Equivalent to ON i.ip = l.ip
WHERE  i.ip IS NULL;

Method 3: EXCEPT

A compact method, but less flexible for complex queries:

SELECT ip 
FROM   login_log

EXCEPT ALL  -- `ALL` preserves duplicates, potentially improving speed
SELECT ip
FROM   ip_location;

Method 4: NOT IN

Use with caution due to potential issues with NULL values:

SELECT ip 
FROM   login_log
WHERE  ip NOT IN (
   SELECT DISTINCT ip  -- `DISTINCT` is optional
   FROM   ip_location
   );

Performance Optimization

Query performance varies depending on data volume and table indexing. NOT EXISTS, LEFT JOIN, and EXCEPT typically offer superior performance. Ensure appropriate indexes are in place on the ip column of both tables for optimal results.

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