Question

I have follow tables:

|ELEMENTS|
------------
|id_element|
|id_catalog|
|value|


|CATALOG|
------------
|id_catalog|
|catalog_name|
|show|
|status|

I tried to add different indecies (several variants):

1) ELEMENT: pair(id_element, id_catalog) and id_element and id_catalog
2) ELEMENT: pair(id_element, id_catalog) and id_element
3) ELEMENT: pair(id_element, id_catalog) and id_catalog
4) ELEMENT: id_element and id_catalog

1) CATALOG: pair(show, status) and id_catalog
2) CATALOG: id_catalog and show and status

Execute follow select:

SELECT DISTINCT `id_element` FROM `ELEMENTS`
       WHERE (id_catalog IN (SELECT `id_catalog` FROM `CATALOG` WHERE status=1 AND show = 1)) limit 10

If there are some rows then it works very fast. But if it is empty – it takes more than 4 sec.

At the same time “SELECTid_catalogFROMCATALOGWHERE status=1 AND show = 1” works fast both there are some rows and empty.

In the table ELEMENTS there are 100.000 records
In the table CATALOG there are 15.000 records

Also I tried “join” but it takes more time than it was before.

Why empty query works so long and what I should do to increase speed rate?

Here are explain answer:

id | select_type          | table                  | type              | possible_keys             | key        | key_len | ref    | rows   | Extra
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1  | 'PRIMARY',           |'ELEMENTS'              | 'index'           | ''                        | null       | null    | null   | 270044 | 'Using where; Using temporary'
2  | 'DEPENDENT SUBQUERY' | 'CATALOG'              | 'unique_subquery' | 'PRIMARY,pair,id_catalog' | 'PRIMARY'  | '4'     | 'func' | 1      | 'Using where'

Answer 1

Well, the reason you’re having the problem is that you’re pulling up the entire catalog database for each request and finding every match between the element and the catalog. If MySQL finds 10 entries, it bails out, but if it never finds them it will continue to check your entire database. I would use an EXISTS query to try and get some performance increase.

SELECT DISTINCT(e.id_element)
FROM ELEMENTS e
WHERE EXISTS (
    SELECT *
    FROM CATALOG c
    WHERE c.id_catalog = e.id_catalog
    AND c.status = 1
    AND c.show = 1)
LIMIT 10;

This will decrease the amount of time MySQL spends looking for the catalog for each element by imposing a LIMIT 1 on the inner query, but you always run the risk of a long search time when there are possibly no matches.