Hi,
> Now as expected it's an dependent subquery and makes use of the index on > document_id. BUT: If we change the "SELECT id" in the subquery to > "SELECT document_id" or "SELECT 1", we get: > > *************************** 1. row *************************** > [...] > *************************** 2. row *************************** > id: 2 > select_type: DEPENDENT SUBQUERY > table: file > type: ref > possible_keys: document_id > key: document_id > key_len: 2 > ref: djbdms.t1.id > rows: 1 > Extra: Using index > > Note the "Using index"! > > I played around with it and it really makes a huge performance > difference - in my case the file table contains a lot of large blobs, is > fragmented and I'm running weak hardware. Seems as if "using index" is > not just (irrelevant or wrong) EXPLAIN output but is what is really > going on? >From the manual : - Using index The column information is retrieved from the table using only information in the index tree without having to do an additional seek to read the actual row. This strategy can be used when the query uses only columns that are part of a single index. So you should save an extra seek from the disk I guess. -- Pooly Webzine Rock : http://www.w-fenec.org/ -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
