MySQL Erratic Query Times

Question

I am using MySQL version 5.5.14 to run the following query from a table of 5 Million rows:

SELECT P.ID, P.Type, P.Name, P.cty
     , X(P.latlng) as 'lat', Y(P.latlng) as 'lng'
     , P.cur, P.ak, P.tn, P.St, P.Tm, P.flA, P.ldA, P.flN
     , P.lv, P.bd, P.bt, P.nb
     , P.ak * E.usD as 'usP' 
FROM PIG P 
  INNER JOIN EEL E 
    ON E.cur = P.cur 
WHERE act='1' 
  AND flA >= '1615' 
  AND ldA >= '0' 
  AND yr >= (YEAR(NOW()) - 100) 
  AND lv >= '0' 
  AND bd >= '3' 
  AND bt >= '2' 
  AND nb <= '5' 
  AND cDate >= NOW() 
  AND MBRContains(LineString( Point(-65.6583, -87.8906)
                            , Point(65.6583, 87.8906)
                            ), latlng) 
  AND Type = 'g' 
  AND tn = 'l' 
  AND St + Tm - YEAR(NOW()) >= '30' 
HAVING usP BETWEEN 300/2 AND 300 LIMIT 100;

On the first occasion, it took 313s, on the second occasion it took 48s and on the third occasion it took 101s. From explain select, the query plan has type: range; key: index and extra: using where on table P followed by type: eq_ref and key: primary on table E. This query does not use query cache. My question is: "Why is the query time so dramatically different?"

Cheers, Ben

Answer 1

Use SQL_NO_CACHE when profiling queries:

SELECT SQL_NO_CACHE P.ID, P.Type, ...

The reason it takes less times on the second run is that the result of the query has been cached by MySQL.

SQL_NO_CACHE disables caching for this query. You can clear the query cache with FLUSH QUERY CACHE.