Question

I have a hug table with 170,000 records.

What is difference between this query

Showing rows 0 - 299 (1,422 total, Query took 1.9008 sec)
    SELECT 1 FROM `p_apartmentbuy` p 
    where 
    p.price between 500000000 and 900000000
    and p.yard = 1
    and p.dateadd between 1290000000 and 1320000000
    ORDER BY `p`.`id` desc
    limit 1669

Explain

And this one:

Showing rows 0 - 299 (1,422 total, Query took 0.2625 sec)
    SELECT 1 FROM `p_apartmentbuy` p 
    where 
    p.price between 500000000 and 900000000
    and p.yard = 1
    and p.dateadd between 1290000000 and 1320000000
    ORDER BY `p`.`id` desc
    limit 1670

Explain:

Both of these queries are using 1 table with same data and have same where clasue, but only limit row count are different

Answer 1

MySQL has a buffer for sorting. When the stuff to be sorted is too big, it sorts chunks, then mergesorts them. This is called “filesort”. Your 1670-th row apparently just overflows the sort buffer.

Read more details here.

Now why it picks another key for the in-memory sort… I am not too sure; but apparently its strategy is not quite good since it ends up being slower.