Skip SQL_CALC_FOUND_ROWS when using post__in whose count matches posts_per_page

[ethitter]

If a post__in array is passed to WP_Query whose length matches the posts_per_page value for the query, SQL_CALC_FOUND_ROWS is unnecessary.

[dlh]

42256.2.diff​ adds a test.

[boonebgorges]

What if the values in post__in point to non-existent posts? Or, perhaps more likely, what if the combination of post__in with some other param (say, a meta_query) makes it so that only a subset of the post__in items are grabbed?

It feels safer to let plugins decide whether they're passing query parameters that guarantee that all and only the items in post__in will end up in the results array. If the plugin knows this, it can pass no_found_rows explicitly.

[dd32]

I'm inclined to agree with @boonebgorges here.

Perhaps including an example use-case and the impact it has upon performance will help sway it from "I don't see the need" to "That seems like a worthwhile shortcut to take".