Does the order of columns matter in a group by clause?

Index Leveraging for Better Performance

Although column order in GROUP BY generally doesn’t impact aggregation results, it becomes rather crucial in terms of performance when utilizing indexes. A query exploiting an index matching the GROUP BY order can significantly maximize the query execution speed:

-- Index to the rescue! The Flash of SQL world:
CREATE INDEX idx_dept_salary ON employees(department, salary);

SELECT department, SUM(salary) FROM employees GROUP BY department;
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Advanced Features Rollercoaster: ROLLUP

When using SQL extensions such as ROLLUP and GROUPING SETS, the scene changes. Now the order of columns in GROUP BY has a role to play as it dictates the hierarchy of subtotals:

-- Plot twist! Changing column order with ROLLUP 
-- gives you a whole new ball game:
SELECT department, job_title, SUM(salary) 
FROM employees 
GROUP BY ROLLUP(department, job_title);


The above code provides subtotals for each job_title within each department, plus a grand total. Swapping columns switches the subtotal hierarchies.

SQL Optimizers: The Puppet Masters

Considering SQL's declarative nature, how a query is written doesn’t drive how the operation is executed. Instead, SQL optimizers interpret the GROUP BY clause and orchestrate the operation to optimize performance, focusing on content, not placement.

All Databases Sing the Same Tune

An interesting aspect is the consistent handling of the GROUP BY clause across diverse database systems. Be it MySQL or PostgreSQL, the underlying principles persist. Thus, the GROUP BY order doesn’t affect the consistency of resultant datasets across systems.

Cardinality: Players off Bench

The cardinality of columns doesn’t influence the final aggregation results in a GROUP BY, but it can affect the row order in the result set when an ORDER BY clause isn't specified. However, this doesn’t alter sums, counts, averages, or any other aggregate computations.

Context is King: Column Order Edition

While it’s widely agreed that GROUP BY order doesn’t influence the grouping outcome, there might be context-specific circumstances, particularly involving complex queries or database-specific extensions, where column order could matter. But remember, these are exceptions, not norms.