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Problems with large MySQL table using ORDER BY and LIMIT

Posted on 2010-09-23
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Last Modified: 2013-12-16
I have a MySQL table that is pretty huge. There are 21 columns and (at this point) about 3 million rows. The interface I'm writing in C# to display data from this table will obviously need the ability to break results into pages. The way I was going to do this was using LIMIT. All results would be sorted by date. So the query for the first page would look like this:

   SELECT * FROM BigTable ORDER BY ItemDate ASC LIMIT 0,250;

This takes forever though, because before it applies the limit it goes through and orders the entire table by the date field. So I'm not really sure how to speed this up and make it work efficiently. Any recommendations?
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Question by:elorc
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Kent Olsen earned 2000 total points
ID: 33746540
Hi elorc,

There are a couple of ways.  :)

Is there an Identity column on the table?  That's an easy solution.

Is the ItemDate column indexed?  Assuming so, try the code below.


Kent

SELECT *
FROM
(
  SELECT ItemDate 
  FROM BigTable 
  ORDER BY ItemDate ASC 
  LIMIT 0,250
) t0
LEFT JOIN BigTable
  ON t0.ItemDate = BigTable.ItemDate;

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by:elorc
ID: 33747852
That makes a ridiculous difference. Thank you.
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Expert Comment

by:Kent Olsen
ID: 33747938
Hi elorc,

Just so you know how this works, MySQL recognizes that there is an index on the ItemDate column and grabs the first 250 dates from the index instead of reading the table.

It then joins only those 250 rows from the table.



Kent
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