How to determine the complexity of an SQL query and its impact on system performance?

The complexity of an SQL query can be assessed based on several parameters: the number and type of joins, the use of subqueries, aggregate functions, indexes, and the volume of data processed. To evaluate the impact on system performance, it is useful to:

• Use EXPLAIN (or EXPLAIN ANALYZE) to analyze the query execution plan — this shows what operations are performed, how many rows are processed, and whether indexes are used.
• Assess the execution time and the load on the database.
• Check if the query blocks other operations.

Optimization may include adding indexes, rewriting the query, limiting the selection, or caching results.