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Asked: 2026-05-11T01:32:39+00:00

So firstly here’s my query: ( NOTE:I know SELECT * is bad practice I

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So firstly here’s my query: (NOTE:I know SELECT * is bad practice I just switched it in to make the query more readable)

SELECT pcln_cities.*,COUNT(pcln_hotels.cityid) AS hotelcount   FROM pcln_cities   LEFT OUTER JOIN pcln_hotels ON pcln_hotels.cityid=pcln_cities.cityid   WHERE pcln_cities.state_name='California' GROUP BY pcln_cities.cityid   ORDER BY hotelcount DESC   LIMIT 5

So I know that to solve things like that you add EXPLAIN to the beginning of the query but I’m not 100% sure how to read the results, so here they are:

alt text http://www.andrew-g-johnson.com/query-results.JPG

Bonus points to an answer that tells me what to look for in the EXPLAIN results

EDIT The cities tables has the following indexes (or is it indices?)

  • cityid
  • state_name
  • and I just added one with both as I thought it might help (it didn’t)

The hotels tables has the following indexes (or is it indices?)

  • cityid
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1 Answer

  1. Hmm, there’s something not very right in your query. You use an aggregate function (count), but you simply group by on id. Normally, you should group on all columns in your select list, that are not an aggregate function.

    As you’ve specified the query now, IMHO, the DBMS can never correctly determine which values he should display for those columns that are not an aggregate …

    It would be more correct if your query was written like:

    select cityname, count(*) from city inner join hotel on hotel.city_id = city_id group by cityname order by count(*) desc

    If you do not have an index on the cityName, and you filter on cityname, it will improve performance if you put an index on that column.

    In short: adding an index on columns that you regularly use for filtering or for sorting, may improve performance. (That is simply put offcourse, you can use it as a ‘guideline’, but every situation is different. Sometimes it can be helpfull to add an index which spans multiple columns. Also, remember that if you update or insert a record, indexes need to be updated as well, so there’s a slight performance cost in adding/updating/deleting records)

    Another thing that could improve performance, is using an inner join instead of an outer join. I don’t think that it is necessary to use an outer join here.

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