What happens inside the SQL server when a query is executed? How is the result obtained from the query text?

When executing an SQL query, the server goes through several stages:

1. Parsing — the query text is broken down into syntactic elements, and correctness is checked.
2. Optimization — a query execution plan is created, indexes are chosen, the order of table joins, etc., to minimize resource costs.
3. Execution — the server accesses data, reads the necessary pages from disk or cache, applies filters, joins tables.
4. Result formation — the obtained data is assembled into the result set and sent to the client.

For example, when executing the query SELECT * FROM Users WHERE Age > 30, the server:

• Parses the query.
• The optimizer decides whether to use an index on age.
• Scans the table or index, selects records.
• Returns the result to the client.

Inside the SQL server, there are components responsible for each stage: parser, query optimizer, executor, and buffer manager for data handling.