DISTINCT requires sorting and deduplication—avoid using it when data is already unique.
Using DISTINCT in SQL queries might seem like an easy way to eliminate duplicates, but it can often lead to unnecessary performance overhead. DISTINCT forces the database to sort and compare entire rows to ensure uniqueness, which becomes expensive when querying large datasets or wide tables.
In many cases, DISTINCT is applied to data that’s already unique by nature—like primary keys, indexed emails, or GROUP BY aggregations—making it redundant. For example, if you're selecting email from a users table where each row is already unique, DISTINCT triggers extra sorting and memory usage for no benefit.
A better approach is to explicitly use GROUP BY if you're aggregating, or to rely on column-level uniqueness when you know the data guarantees it. This keeps the query planner from overworking and allows indexes to operate more efficiently, especially on large-scale analytics or reporting queries.
Galaxy’s optimization engine highlights inefficient use of DISTINCT and recommends rewrites when data already meets uniqueness constraints. Removing it can result in 1.5x to 3x speed gains—especially in reporting queries, dashboards, or OLAP environments where sort and memory costs compound quickly.
SELECT DISTINCT email FROM users;
SELECT email FROM users GROUP BY email;
1.5–3x faster if DISTINCT avoided
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