<http://gtalug.org/meeting/2018-02/> # Postgres, MVCC, and You (or, why COUNT(*) is slow) with David Wolever It's hard to be a developer today without using a database… but they're often surrounded by an air of reverent mystery. One of those mysteries is why it's so slow to count all the rows in a table using COUNT(*). After all, it's just a matter of walking a b-tree and counting leaves… and that should be trivial to optimize! In this talk I'll answer the question of "why COUNT(*) is slow" by taking a deep dive into the the internals of Postgres' MVCC implementation, looking at: The question of "why COUNT(*) is slow" The on-disk storage layout and why, under the hood, it's not a b-tree What Postgres means by MVCC, with examples of the utility of transactions Introduce xid, xmin, xmax Discuss tuple visibility Explain VACUUM and xid wraparound Show off transaction isolation levels This talk is accessible to anyone who's used an SQL database, with enough depth that experienced developers will find some interesting tidbits. About me: David is a Pythonista from Toronto, Canada. He's co-founder of both PyCon Canada – Canada's regional Python conference – and Akindi.com – a small company that's making multiple-choice bubble sheet tests a little bit less terrible. He's also the author of nose-parameterized, a parameterized testing for every Python testrunner, and pprint++, a Python pretty-printer that's actually pretty. Say hi on Twitter: @wolever! ## Location George Vari Engineering and Computing Centre 245 Church Street, Room 203 Ryerson University <http://goo.gl/maps/16oJ2> <http://www.openstreetmap.org/way/23447525> ## Schedule * 6:00 pm - Please discuss on the general mailing list (i.e. <talk@gtalug.org>) where you want to go for dinner. * 7:30 pm - Meeting and presentation. * 9:00 pm - After each meeting, a group of GTALUGers move to The Imperial Pub (54 Dundas St East) for refreshments and more socialising. # Code of Conduct We want a productive happy community that can welcome new ideas, improve every process every year, and foster collaboration between individuals with differing needs, interests and skills. We gain strength from diversity, and actively seek participation from those who enhance it. This code of conduct exists to ensure that diverse groups collaborate to mutual advantage and enjoyment. We will challenge prejudice that could jeopardise the participation of any person in the community. The Code of Conduct governs how we behave in public or in private whenever the Linux community will be judged by our actions. We expect it to be honoured by everyone who represents the community officially or informally, claims affiliation or participates directly. It applies to activities online or offline. We invite anybody to participate. Our community is open. Please read more about the GTALUG Code of Conduct here: <http://gtalug.org/about/code-of-conduct/>. If you have any questions, comments, or concerns about the GTALUG Code of Conduct please contact the GTALUG Board @ <board@gtalug.org>.