Temel sorgu yürütme iş akışı

Run SELECT * FROM users and PostgreSQL follows a set query execution workflow to turn SQL into results. Knowing it helps you write faster queries.

The query workflow has four stages: parsing your SQL, rewriting it (e.g. expanding views), planning the best execution path, then executing the chosen plan.

Stage 1 is parsing: PostgreSQL checks your SQL's syntax, breaks it into tokens, and builds a parse tree. Any typo throws an error right here.

During parsing, PostgreSQL reads which columns you want, from which table, under which condition. The query below is a simple example to parse.

Stage 2 is rewriting: PostgreSQL's rule system expands views into their underlying queries and applies any custom rules, producing a query tree ready for planning.

Stage 3 is the query planner — the most crucial step. It weighs table sizes, indexes, and data distribution to find the plan with the lowest estimated cost.

The planner generates several paths — seq scan vs index scan, different join orders — estimates each one's cost, and picks the cheapest as the execution plan.

Stage 4 is the executor: it runs the chosen plan, fetching data, filtering, sorting, and joining, then returns results — step by step in sequence.

The full query flow: SQL goes through parser, rewriter, planner, and executor to reach results. Knowing it is key to debugging slow queries.

You've learned about the PostgreSQL query execution workflow. Let's see if you can identify the correct order of the main stages.

Recap: PostgreSQL processes a query through parsing, rewriting, planning, and execution. Next you'll read execution plans with EXPLAIN.