Can Postgres Optimize Groupwise MAX Queries Without a Full Table Scan?

Can Postgres Optimize Groupwise Maximum Queries Without Scanning All Rows?

Postgres lacks a direct approach to maximize group queries efficiently, as evident from the expensive full table scans observed in the given query.

SOLUTION

To avoid excessive scans, consider utilizing a lookup table for options and linking it to the records table via option_id. Establishing a foreign key constraint between these tables would maintain referential integrity.

SQL FOR LOOKUP TABLE CREATION:

CREATE TABLE options (
  option_id int PRIMARY KEY
, option    text UNIQUE NOT NULL
);

Data Insertion from Existing Records Table:

INSERT INTO options
SELECT DISTINCT option_id, 'option' || option_id
FROM   records;

Optimized Query using Subquery:

SELECT option_id, (SELECT max(id)
                   FROM   records
                   WHERE  option_id = o.option_id) AS max_id
FROM   options o
ORDER  BY 1;

Alternatively, an optimized subquery can retrieve the maximum id:

SELECT option_id, (SELECT id
                   FROM   records
                   WHERE  option_id = o.option_id
                   ORDER  BY id DESC NULLS LAST
                   LIMIT  1) AS max_id
FROM   options o
ORDER  BY 1;

Optimal Index for Query Performance:

CREATE INDEX ON records (option_id, id DESC NULLS LAST);

This optimized approach significantly improves query execution by utilizing index scans or even index-only scans for both the lookup table and the primary table, resulting in superior performance compared to full table scans.

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