MySQL index usage on join - mysql

Question

I know there are several questions similar to this one, but those I've found do not relate directly to my problem.
Some initial context: I have a facts table, called ft_booking, with around 10MM records. I have a dimension called dm_date, with around 11k records, which are dates. These tables are related through foreign keys, as usual. There are 3 date foreign keys in the table ft_booking, one for boarding, one for booking, and other for cancellation. All columns have the very same definition, and the amount of distinct records for each is similar (ranging from 2.5k to 3k distinct values in each column).
There I go:
EXPLAIN SELECT
*
FROM dw.ft_booking b
LEFT JOIN dw.dm_date db ON db.sk_date = b.fk_date_booking
WHERE date (db.date) = '2018-05-05'
As you can see, index is being used in the table booking, and the query runs really fast, even though, in my filter, I'm using the date() function. For brevity, I'll just state that the same happens using the column fk_date_boarding. But, check this out:
EXPLAIN SELECT
*
FROM dw.ft_booking b
LEFT JOIN dw.dm_date db ON db.sk_date = b.fk_date_cancellation
WHERE date (db.date) = '2018-05-05';
For some mysterious reason, the planner chooses not to use the index. Now, I understand that using some function over a column kind of forces the database to perform a full table scan, in order to be able to apply that function over the column, thus bypassing the index. But, in this case, the function is not over the actual foreign key column, which is where the lookup in the booking table should be ocurring.
If I remove the date() function, the index will be used in any of those columns, as expected. One might say, then, "well, why don't you just get rid of the date() function?" - I use metabase, an Interface which allow users to use a graphical interface in order to build queries without knowing MySQL, and one of the current limitations of that tool is that it always uses the date() function when building queries not directly written in MySQL - hence, I have no way to remove the function in the queries I'm running.
Actual question: why does MySQL use index in the first two cases, but doesn't in the latter, considering the amount of distinct values is pretty much the same for all columns and they have the exact smae definition, apart from the name? Am I missing something here?
EDIT: Here is the CREATE statment of each table involved. There are some more, but we just need here tables ft_booking and dm_date (first two tables of the file).

Answer 1

You are "hiding date in a function call". If db.date is declared a DATE, then
date (db.date) = '2018-05-05'
can be simply
db.date = '2018-05-05'
If db.date is declared a DATETIME, then change to
db.date >= '2018-05-05'
AND db.date < '2018-05-05' + INTERVAL 1 DAY
In either case, be sure there is an index on db.date.
If by "I have a dimension called dm_date", you mean you built a dimension table to hold just dates, and then you are JOINing to the main table with some id, ... To put it bluntly, don't do that! Do not normalize "continuous" things such as DATE, DATETIME, FLOAT, or other numeric values.
If you need to discuss this further, please provide SHOW CREATE TABLE for the relevant table(s). (And please use text, not screen shots.)
Why??
The simple answer is that the Optimizer does not know how to unravel any function. Perhaps it could; perhaps it should. But it does not. Perhaps the answer involves not wanting to see how the function result will be used... comparing against a DATE? against a DATETIME? being used as a string? other?
Still, I suggest the real performance killer is the existence of dm_date rather than indexing and using the date in the main table.
Furthermore, the main table is bigger than it needs to be! fk_date_booking is a 4-byte INT SIGNED instead of a 3-byte DATE.