perf: cap max count on list views

[ankush]

Problem: In InnoDB counting is essentially O(N) operation, it can be pretty fast on indexes so it doesn't feel like it most of the time but when filters don't use any index it usually means doing a full table scan.

get_count accounts for 4% of all requests on FC - #21742 . This isn't even considering outliers that were removed from data. Users with large DB often need to disable count on list views for site to stay usable.

This makes Count sad.

Fix: Adding a limit will stop the scan as soon as that many records are matched.

Implementation:

• We only show accurate count up to 1000 values now
• User can click on the count to get an accurate count if really required. Most users don't care about this at all!

Outcome:

• If filters don't match more than 1k records then this PR makes no difference.
• If filters match more than 1k records then all that extra cost is now not paid. So for site with millions of matching records get_count will finish much faster. The main benefit is user won't NEED to disable the count.

Count is happy now.

[ankush]

Todo

• tests
• actual count formatting
• child table (?)
• avoid select star expansion
• report view

[Tom-Finke]

I got an error in one of my List Views and traced it to this change.
In Frappe Console do

import frappe.desk.reportview
frappe.local.form_dict = {"doctype": "User", "distinct": "true", "limit": 1001}
frappe.desk.reportview.get_count()

This gives DoesNotExistError: DocType tinct `tabUser` not found

[ankush]

#25511 should handle it.