Question

I have the following query that when executed, it just runs for hours and eventually the server times out.

SELECT l.mloc_id, COUNT( l.tabc_id ) , l.tabc_id, e.establishment_id, e.name, l.LocationName, l.naics, l.cuisine, l.cuisine2, l.service, l.www, l.owner, l.email, l.status
FROM vg_ydrink2.m_loc AS l
JOIN vg_ydk.permit_beverage AS pb ON ( l.tabc_id = pb.permit_code
AND pb.create_date = ( 
SELECT MAX( create_date ) 
FROM vg_ydk.permit_beverage
WHERE permit_code = l.tabc_id ) ) 
JOIN vg_ydk.establishment AS e ON ( pb.establishment_id = e.establishment_id ) 
GROUP BY l.mloc_id
ORDER BY  `l`.`tabc_id` ASC 
LIMIT 1 , 30

No errors are reported. It simply never gets completed.

The problem lies in the sub-select, because when removing it, the query executes without a problem. I also don’t think it’s the group function causing the issue, because I have tried keeping the subquery but removing the MAX and that did not work either.

Any thoughts?
Thank you in advance.

***EDIT: After having played with the create_date index a bit, I realized that due to the nature of its values, its cardinality is constantly very low. For that reason, it is not treated as an index by the query and as a result, too many rows are constantly loaded.
I have since changed the query altogether, so this is not an issue anymore. If anybody comes across a similar issue, make sure the used field is indexed properly. Analyze/optimize/repair your table if need be.

M.

Answer 1

Try this variation:

select l.mloc_id,
    COUNT(l.tabc_id),
    l.tabc_id,
    e.establishment_id,
    e.name,
    l.LocationName,
    l.naics,
    l.cuisine,
    l.cuisine2,
    l.service,
    l.www,
    l.owner,
    l.email,
    l.status
from vg_ydrink2.m_loc as l
inner join (
    select permit_code,
        max(create_date) as MaxCreateDate
    from vg_ydk.permit_beverage
    group by permit_code
    ) pbm on l.tabc_id = pbm.permit_code
inner join vg_ydk.permit_beverage as pb on pbm.permit_code = pb.permit_code
    and pbm.MaxCreateDate = pb.create_date
inner join vg_ydk.establishment as e on pb.establishment_id = e.establishment_id
group by l.mloc_id
order by `l`.`tabc_id` asc 
LIMIT 1,
    30