Fix Relation#exists? queries with query cache

[kamipo]

If a connection adapter overrides select_* methods, query caching will
doesn't work. This patch changes select_value to select_one in
Relation#exists? to ensure query caching.

Fixes #29449.

[matthewd]

I wonder whether other callers are expecting select_value to bypass the query cache.

[vipulnsward]

👍 for the second change. Does it not have an overhead?

[kamipo]

select_{value,values,rows} in mysql2 and postgresql adapters are for bypassing ActiveRecord::Result instanciation.
These are mostly used in schema statements (select_all in Relation is almost all, select_one and select_value are once) and it is not a performance hotspot in AR internal (actually select_one in mysql2 adapter was removed in #25507).
I think that removing the bypassing is acceptable.

[tjschuck]

It seems like this can be cleanly and safely backported to 5-0-stable and 5-1-stable as well with no change in behavior, so I'd like to preemptively request that here!

[zmoazeni]

Thank you @kamipo for jumping on this. I for one really appreciate all the hard work all of you put into these performance updates. I've tried following along on several of the PRs (like @matthewd's awesome connection pool updates), but it is definitely a different league.

One thing I was going to check was to see if other typical Rails queries are relying on select_one but it looks like you have tests for those cases, and that's awesome.

As for:

I wonder whether other callers are expecting select_value to bypass the query cache.

People can currently opt out of the query cache right now yeah?

> Project.cache { Project.uncached { 3.times { Project.find(1) } }}
  Project Load (0.3ms)  SELECT  `projects`.* FROM `projects` WHERE `projects`.`id` = 1 LIMIT 1
  Project Load (0.3ms)  SELECT  `projects`.* FROM `projects` WHERE `projects`.`id` = 1 LIMIT 1
  Project Load (0.2ms)  SELECT  `projects`.* FROM `projects` WHERE `projects`.`id` = 1 LIMIT 1

My vote would be that the programmer should be explicit by wrapping their queries in uncached { ... } blocks if they want to avoid the cache.

There could also exist other scenarios that I'm not aware of, but that feels appropriate with the bits that I do.