Question

I just changed from Access to mySQL. I have a few tables, called “Veranstaltung” and “Veranstaltung_User”. I want to get all events (=Veranstaltung) to which users have subsribed, exept the one, the user is viewing.

I did this with the following statment

SELECT VID, Bezeichnung, Datum 
FROM tbl_Veranstaltung 
WHERE year(Datum) = 2012 
    AND VID <> 613 
    AND VID IN (SELECT BewerbID 
                FROM Veranstaltung_User 
                WHERE UserID IN (SELECT UserID 
                                FROM Veranstaltung_User 
                                WHERE BewerbID = 613) 
                    AND UserID <> 0) 
ORDER BY Datum

In Access, this query was quite fast – in mySQL it needs 44,56 seconds – too much 😉

how can I optimize my statement for faster quering? (mySQL > 5, tables ware MyISAM)

thanks for help

Answer 1

Two changes:
1. Uses JOINs instead of nested IN clauses
2. Uses <= and > instead of the YEAR() function

The advantage of change 1 is that sometime IN performance can be worse than expected, and nesting them will exascerabate that.

The advantage of change 2 is that it allows an index seek to find a range of records. Where-as using YEAR() forcfes a scan of the entire table/index.

Both of which assume you have appropriate indexes.

SELECT
  V.VID, V.Bezeichnung, V.Datum
FROM
  tbl_Veranstaltung     V
INNER JOIN
  Veranstaltung_User    VUser
    ON VUser.BewerbID = V.VID
INNER JOIN
  Veranstaltung_User    VUser2
    VUser.UserID = VUser2.UserID
WHERE
      V.Datum >= '20120101'
  AND V.Datum <  '20130101'
  AND V.VID <> 613
  AND VUser.UserID <> 0
  AND VUser2.BewerbID  = 613
ORDER BY
  Datum