How to Read a SQL Execution Plan and Distinguish the Cause of a Slow Query from a Symptom
An execution plan does not explain a delay by itself: it shows the way the optimizer chose to process the data. To find the cause, you must match the estimates against the metrics of the specific execution, find the first significant cardinality gap before the growth in work, account for repetitions of operators, and check I/O, temporary data, and waits outside the plan. The article gives a step-by-step analysis method for PostgreSQL, SQL Server, Oracle Database and MySQL 8.x.