Re: Strangeness with UNIQUE indexes and UTF-8

On Sun, Jun 06, 2021 at 03:54:48AM -0700, Omar Kilani wrote:
> What I sort of don't get is... before we insert anything into these
> tables, we always check to see if a value already exists. And Postgres
> must be returning no results for some reason. So it goes to insert a
> duplicate value which somehow succeeds despite the unique index, but
> then a reindex says it's a duplicate. Pretty weird.

In addition to the other issues, this is racy.

You 1) check if a key exists, and if not then 2) INSERT (or maybe you UPDATE if
it did exist).

https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use

Maybe you'll say that "this process only runs once", but it's not hard to
imagine that might be violated. For example, if you restart a multi-threaded
process, does the parent make sure that the child processes die before itself
dying? Do you create a pidfile, and do you make sure the children are dead
before removing the pidfile ?

The right way to do this since v9.6 is INSERT ON CONFLICT, which is also more
efficient in a couple ways.

--
Justin