On Mon, Nov 02, 2020 at 03:08:12PM -0500, Tom Lane wrote: >Philip Semanchuk <philip@americanefficient.com> writes: >> The query I asked about in the original post of this thread has 13 relations in it. IIUC, that's 13! or > 6 billion possibleplans. How did the planner pick one plan out of 6 billion? I'm curious, both for practical purposes (I want my queryto run well) and also because it's fascinating. > >The twenty-thousand-foot overview is > >https://www.postgresql.org/docs/devel/planner-optimizer.html > >and then ten-thousand-foot level is the planner README file, > >https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob_plain;f=src/backend/optimizer/README;hb=HEAD > >and then you pretty much gotta start reading code. You could also dig >into various planner expository talks that people have given at PG >conferences. I don't have links at hand, but there have been several. > Yeah. The jump from high-level overviews to reading source code is a bit brutal, though ... FWIW a short list of relevant talks I'm aware of & would recommend: * Explaining the Postgres Query Optimizer [Bruce Momjian] https://www.postgresql.org/files/developer/tour.pdf * Intro to Postgres Planner Hacking [Melanie Plageman] https://www.pgcon.org/2019/schedule/events/1379.en.html * Learning to Hack on Postgres Planner [Melanie Plageman] https://www.pgcon.org/2019/schedule/attachments/540_debugging_planner_pgcon2019_v4.pdf * What’s in a Plan? [Robert Haas] https://www.postgresql.eu/events/pgconfeu2019/schedule/session/2741-whats-in-a-plan/ * A Tour of PostgreSQL Internals [Tom Lane] https://www.postgresql.org/files/developer/tour.pdf * Inside thePostgreSQL Query Optimizer [Neil Conway] http://www.neilconway.org/talks/optimizer/optimizer.pdf Some are a bit dated, but the overall principles don't change much. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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