>I have a simple query: > >SELECT first 1 ITEM_DATE FROM HISTORY >WHERE ITEM_ID = 'ITEM_ID_123456' >and ITEM_TYPE = 'TYPE1' >AND ITEM_STATUS > 200 and ITEM_STATUS <> 310 >ORDER BY ITEM_DATE ASC > >Which runs in about 1.5 seconds. If I remove the Order by it's less than >100ms. There are only three records in the result set. >There is an ascending index on ITEM_DATE and the statistics are up to date. > >Does anyone know why there would be such a difference in performance when >using\not using the order by? What is the >engine ordering? It can't be the three records to take 1.5 seconds, surely?
Probably the PLAN is different, I assume you have an index on ITEM_DATE and Firebird wrongly believes it would could speed up the query by using it (it doesn't know that only three rows will be returned, I guess ITEM_ID isn't a very selective field). Change your query to: SELECT first 1 ITEM_DATE+0 FROM HISTORY WHERE ITEM_ID = 'ITEM_ID_123456' and ITEM_TYPE = 'TYPE1' AND ITEM_STATUS > 200 and ITEM_STATUS <> 310 ORDER BY 1 ASC and the speed should be closer to 100ms again. HTH, Set
