Generating linkage data gets expensive with includes

[st0012]

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This is not a feature bug, but a potential performance issue.

It's very common to eager load data in JR. But if we keep includes values when plucking for linkage data, it can make the query unnecessarily expensive.

Let's use Post model for example:

If we directly pluck all posts' created_at column, the query would be very simple

SELECT "posts"."created_at" FROM "posts"

But if we keep includes values in the previous relation object, like

Post.includes(:author).pluck(:created_at)

It generates unnecessarily complicated query

SELECT "posts"."created_at" FROM "posts" LEFT OUTER JOIN "people" ON "people"."id" = "posts"."author_id"

This is how we pluck polymorphic to_many linkage

assoc = source.public_send("records_for_#{relationship.name}", options)
......
assoc.pluck(:type, :id)

And we do apply includes in records_for_xxxx method

      define_on_resource associated_records_method_name do |options = {}|
        .....

        records = resource_klass.apply_includes(records, options)

This means our pluck query will be very expensive due to the previous includes