Where statement after a UNION in SQL?

SQL Performance Best Practices: UNION Edition

Indexing and Subquery Proficiency

Indexes can be your best bet to accelerate JOINs used in a UNION. It trims down the database engine's search space, supercharging JOIN operations impacting your UNION execution.

Subqueries can pre-filter unwanted rows from each select statement in UNION operation. This lightens your processing load, making your query faster and simplicity your friend:

// Who needs baggage, travel light!
SELECT * FROM (
    SELECT a FROM t1 WHERE a > 50
    UNION
    SELECT b FROM t2 WHERE b < 25
) AS sub WHERE sub.a = 'filter_value';


Journey through Query Analysis and Temporary Tables

Utilize EXPLAIN to know how your UNION query gears up. Peering into the execution process can reveal bottlenecks and guide your optimization strategies.

When UNIONs get thick, consider temporary tables to store interim results. Break your query down to manageable tasks.

Reducing Data Traffic: Polished Practices

Using SELECT * can be tempting, but be specific in your column selection. It not only clears the clutter but also reduces data traffic and optimizes performance.

Also, take advantage of your DBMS's query caching capability for recurring UNION operations. Less stress for your processor, quicker results for you!

In pagination scenarios or whenever possible, employ LIMIT to fetch a specific number of rows sans unnecessary data retrieval.

A Dip into Advanced SQL Approaches

Stored procedures and User Defined Functions (UDFs) could encase intricate UNION logic to streamline code and ensure maintainability.

For larger datasets, database partitioning acts as a performance booster by running UNIONs on a reduced data subset.

Unlocking Turbo Boost for SQL Performance

Regular Database Checkups and Surgery

Persistent examination and refinement for your database schema and index strategy can greatly optimize UNION query performance. Indexing opportunities and schema tweaks can, and often do, make big strides in performance.

Test, Dust Off, Repeat

Benchmark the performance of various UNION options. You may find that a WHERE clause before or after the UNION can significantly affect performance, but it's not always clear which is optimal. Data doesn't lie - confirm with tests.

Continuous SQL Calibration

In SQL Performance Optimization, there's always room for improvement. Continuous learning and application of new strategies, techniques like optimized subqueries, effective use of indexing, and active caching will testify to your progress in action.