How to use LAST_DAY() in MySQL so index advantage is not lost?

I'm using MySQL tables.

Sample table EMP (primary_key on (EID, DOJ)) :

ENAME	EID	DEPT	SAL	DOJ (YYYY-MM-DD)
A	6	ee	2000	2021-03-01
B	5	me	2020	2021-04-30
C	3	it	2000	2020-12-27
D	4	cv	2020	2020-10-31
E	1	it	2000	2021-01-01
F	2	it	null	2021-02-28
G	7	ee	null	2020-11-20

I have one job that deletes and inserts data into a similarly structured table.

The process needs to delete non-monthly data. That is, it will delete rows with EID 6, 3, 1, 7: These rows have DOJ that is not month-end.

The query I came up with:

-- DOJ is of type DATE
DELETE FROM EMP WHERE LAST_DAY(DOJ) <> DOJ ;

It works as expected. But, because there is large data (~5 million) this is slow.

I understand that because of LAST_DAY() function, I'm loosing the index advantage of DOJ. Can you please suggest how can I improve the query?

Solution

You are right about the index. It is no use here. The only idea that comes to mind is a generated column to tell you about the date being the month's end or not. Thus you'd have a column you can index and use in your query:

create table emp 
(
  ename varchar(100),
  ...
  doj date,
  is_month_end bool as (doj = last_day(doj))
);

create index idx_month_ends on emp (is_month_end);

delete from emp where not is_month_end;

Demo: https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=397388b70bb1f459bbefce630ad27ac4

An index can only help, though, if this is about a very small part of the data in the table, say 1%. With many more rows to delete it makes more sense to read the whole table.