Question

I have 4000 rows for example, and I define X limit.

The query stops after it finds X rows? or the query finds all the rows and then takes X rows from the found rows?

Thank you.

Answer 1

From MySQL Reference Manual:

If you use LIMIT row_count with ORDER BY, MySQL ends the sorting as
soon as it has found the first row_count rows of the sorted result,
rather than sorting the entire result. If ordering is done by using an
index, this is very fast. If a filesort must be done, all rows that
match the query without the LIMIT clause must be selected, and most or
all of them must be sorted, before it can be ascertained that the
first row_count rows have been found. In either case, after the
initial rows have been found, there is no need to sort any remainder
of the result set, and MySQL does not do so.

So it looks like it’s possible that the entire result set is known before the LIMIT is applied. But MySQL will try everything it can not to do so. And you can help it by providing useful indexes that match your queries.

EDIT: Furthermore, if the set is not sorted it terminates the SELECT operation as soon as it’s streamed enough rows to the result set.