Debug/visualize database query internal mechanism

I wonder if some open-source SQL database servers have a possibility, how to find out (maybe even in graphical representation), what actually happened inside during the query (e.g. whether table scan was used, or if and which index(es) were 开发者_StackOverflow社区used..) step-by-step. It would be useful for database optimization.

Most servers have some sort of way to display a query execution plan. Explain query in mysql, for instance. Which server are you using?

Most all will have tools/commands to describe query plans, the graphical part you may have to pay for.