Why this query is fast

Welcome to another database question. In this question I created a community poll question and provided some answers. All answers can be correct of course but the question is what is the most efficient? this is what I try to explore in this video and compare how different database platforms such as mysql or Postgres differ in those implementations;
Table T with three integer fields A, B and C.
A has a primary key clustered index.
B has a secondary index.
Which query is more likely to be the most efficient?
0:00 Intro
1:00 The Question
3:20 WHERE C BETWEEN 50,50000
5:30 WHERE B BETWEEN 50,50000
13:20 WHERE A BETWEEN 50,50000
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[@peterhahne2035]

Am I missing something or is uniqueness a game breaker here? Since B is not a primary key we have no guarantee on the uniqueness and therefore we might have millions of records between 50 and 50000

[@hnasr]

That is a great point for sure 👍

[@HarshKapadia]

Oh good point! I didn't even think of that. I was only thinking in terms of the index.

[@joseortiz_io]

Good stuff my guy. It’d be great to have some visuals with the type of discussions just to bring the point home. Anyway, I appreciate these videos a lot. Thanks Hussein! 😁

[@fastestwaydown]

It's 'analyze table' in PostgreSQL, mostly paired with vacuum as far as I use it :)

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[@HarshKapadia]

This was so much fun!

[@denifortran5482]

nice video, can you make one with the books from bookshelve explaining them?

[@owenflanagan3997]

pretty sure in postgres ANALYZE updates table stats

[@ruchikaahujaasm]

@hnasr , if possible can you please do some live coding for building backend systems?

[@MartinsTalbergs]

We can explore internals on primary index built from multiple columns, maybe there is something interesting there

[@weathercontrol0]

I think that it's built with just concatenated values of columns

[@MartinsTalbergs]

@@weathercontrol0 how would you concatenate different data types, like date and uint and enum?

[@weathercontrol0]

@@MartinsTalbergs after all, it's all just bytes, I don't think there's a problem concatenating different data types

[@MartinsTalbergs]

@@weathercontrol0 there should be a problem, because an index has to be built... Naively, sure - they could do basic BTREE of zeroes and ones

[@susmitvengurlekar]

IF, A was not a primary key but had a clustered index , then what ? Once you find page for value 50 by doing a primary index scan for the value you got from A's index, every consecutive items in the page and the consecutive pages will have the required data (since it's clustered). BUT, is the database smart enough to know that it doesn't need to scan the primary index for each of the values which it finds in A's index ?

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First to comment

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[@gyroninjamodder]

There's a typo in your chapter titles btw

[@hnasr]

Thanks fixed!

[@lqx7]

Early