Part IV: Planning

How does an engine decide what to run for a query, from its text alone?

Every plan in Parts I to III was written by hand, from DuckDB’s EXPLAIN. An engine has only the query’s text. This part writes the planner that turns the text into a plan: it reads the text, checks every name in it, and builds the plan the text describes in the plainest way, then measures what the plainest way costs (ch11). Then it teaches the planner rules that rewrite the plan into a cheaper one with the same rows, whatever the data, and finds they rebuild every plan the book wrote by hand (ch12). Last, it chooses between plans that give the same rows by estimating what each will cost, from what the files say about themselves (ch13).

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