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Query performance on indexed columns
Posted by: marco rossi
Date: April 24, 2007 06:52AM

I've got a question regarding the performance of queries on indexed columns.

I've got a query like this
SELECT SUM (value) FROM table WHERE id=x
id is an index for the table 'table'
In order to excute this query, I think the Db only examines the rows for wich the query WHERE condition is true, without reading all the rows in the table. Am I right?

So, if I got an INNO_DB table with say 50000 records with articles identified by IDs, and say that every article is repeated in the table a maximum of 5 times, issuing a SELECT SUM query on the indexed ID field, will not impact on overall performance cause only a maximum of 5 rows will be examined.
In such a situation, I can say that the SELECT SUM performance is quite similar to the performance af a simple SELECT query, isn't it?

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