Engineering/Database

How to Read SQL Execution Plans with EXPLAIN

When a SQL query is slow, the first instinct is often to add an index.That may solve the problem, but it may also miss the real cause. The query might already have a usable index that the optimizer decided not to use. The join order may be inefficient, the optimizer may have estimated the wrong number of rows, or the database may be sorting a large intermediate result.Consider the following quer..