I wish we got a yellow bang on execution plans when SQL Server was making up an estimate out of thin air.
I’ll give you an example: if you compare two columns on the same table, looking to find rows where they’re equal, SQL Server has a hard-coded estimate that 10% of the rows will match.
I’ll query the Stack Overflow database’s Users table to find people whose DisplayName is the same as their Location. To give SQL Server a fighting chance at estimation, I’ll create indexes on both columns, in both orders, just to preemptively head off the folks