Re: Fully documenting the design of nbtree row comparison scan keys

пт, 31 окт. 2025 г. в 00:35, Peter Geoghegan <pg(at)bowt(dot)ie>:

> I didn't understand every nuance of row compare inequalities myself
> until quite recently. The rules with NULLs are particularly tricky.
>
> It seems worthwhile to clear things up now in large part due to the
> recent addition of code in places like _bt_advance_array_keys -- code
> that wants to to treat row compare keys as if they were just a simple
> scalar inequality on the row compare's most significant column. That
> general behavior isn't new (e.g., _bt_first has long ignored row
> compare scan key markings when deducing a NOT NULL constraint), but
> it's not easy to see why it's correct.
>

Greetings.

I took a look at the patch. Proposed comments look highly valuable,
especially around NULLs, doesn't look immediately obvious, so
definitely requires a comment.
Looks good to commit.

Wouldn't it be good to add such information also into the user
documentation, say into
https://www.postgresql.org/docs/current/functions-comparisons.html#ROW-WISE-COMPARISON
?

--
Victor Yegorov