Don't eager load when plucking with includes/limit

[dvandersluis]

Summary

Consider a Company c that has many Contracts, and the following statement:

Company.where(id: c.id).includes(:contracts).limit(1).pluck('contracts.id')

The pluck runs two queries, one to get distinct values of the main record primary key, which has the LIMIT applied, and a second to get the pluck values, with is filtered by the IDs retrieved in the first query:

SELECT DISTINCT "companies"."id" FROM "companies" LEFT OUTER JOIN "contracts" ON "contracts"."company_id" = "companies"."id" WHERE "companies"."id" = ? LIMIT ?  [["id", 12], ["LIMIT", 1]]
SELECT "contracts"."developer_id" FROM "companies" LEFT OUTER JOIN "contracts" ON "contracts"."company_id" = "companies"."id" WHERE "companies"."id" = ? AND "companies"."id" = ?  [["id", 12], ["id", 12]]

The limit is applied only to the number of records retrieved only from the first query, which means that in the pluck above, multiple results are returned, despite asking for 1.

As well, running a second query has a performance impact, especially if the query isn't hitting an appropriate index (as I found in my production app). In the best case scenario, it's still a second query for no benefit. Since a pluck statement isn't going to instantiate any AR objects, nothing will be preloaded anyways.

This change makes includes.limit.pluck behave identically to left_joins.limit.pluck (same queries, same results).

Fixes #36217.

[dvandersluis]

I'm not sure if this test is needed or useful, so I'm happy to remove it if desired.

[rails-bot]

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