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POIs it bad practice to do "small_table LEFT JOIN large_table"?
 

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  1. POIs it bad practice to do "small_table LEFT JOIN large_table"?
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    <p>I'm connecting to a database which I cannot administer and I wrote a query which did a left join between two tables - one small and one several orders of magnitude larger. At some point, the database returned this error:</p> <blockquote> <p>Incorrect key file for table '/tmp/#sql_some_table.MYI'; try to repair it</p> </blockquote> <p>I contacted the administrators and I was told that I'm getting this error because I'm doing the left join incorrectly, that I should NEVER left join a small table to a large table and that I should reverse the join order. The reason they gave was that when done my way, MySQL will try to create a temp table which is too large and query will fail. Their solution fails elsewhere, but that's not important here.</p> <p>I found their explanation odd, so I ran explain on my query:</p> <pre><code> id = '1' select_type = 'SIMPLE' table = 'small_table' type = 'ALL' possible_keys = NULL key = NULL key_len = NULL ref = NULL rows = '23' Extra = 'Using temporary; Using filesort' id = '1' select_type = 'SIMPLE' table = 'large_table' type = 'ref' possible_keys = 'ID,More' key = 'ID' key_len = '4' ref = 'their_db.small_table.ID' rows = '41983' Extra = NULL </code></pre> <p>(The 41983 rows in the second table are not very interesting to me, I just needed the latest record, which is why my query has <code>order by large_table.ValueDateTime desc limit 1</code> at the end.)</p> <p>I was careful enough to do a select by columns which the admins themselves told me, should hold unique values (and thus I assumed indexed), but it seems they haven't indexed those columns.</p> <p>My question is - is doing the join the way I did ('small_table LEFT JOIN large_table') bad practice in general, or can such queries be made to execute successfully with proper indexing?</p> <p>Edit: Here's what the query looks like (this is not the actual query, but similar):</p> <pre><code>select large_table.ValueDateTime as LastDate, small_table.DeviceIMEI as IMEI, small_table.Other_Columns as My_Names, large_table.Pwr as Voltage, large_table.Temp as Temperature from small_table left join large_table on small_table.ID = large_table.ID where DeviceIMEI = 500 order by ValueDateTime desc limit 1; </code></pre> <p>Basically what I'm doing is trying to get the most current data for a device, given that Voltage and Temperature change over time. The DeviceIMEI, ID and ValueDateTime should be unique, but aren't indexed (like I said earlier, I don't administer the database, I only have read permissions).</p> <p>Edit 2:</p> <p><strong>Please focus on answering my actual question, not on attempting to rewrite my original query.</strong></p>
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    1. USNikola Novak
    1. USNikola Novak
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      1. POIs it bad practice to do "small_table LEFT JOIN large_table"?
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      1. VTUpMod
    2. VO
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    1. COAnother example why MySQL is ALMOST a good RDMS... And I can't believe that admin could said something soooo stupid: "reverse join order". Yeah, sure.
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      1. USKuba Wyrostek
    2. CONikola, how many rows do you get withouth this `limit 1`?
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      1. USKuba Wyrostek
    3. CO@KubaWyrostek If I remove the limit 1, I get 200652 rows in result.
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      1. POIs it bad practice to do "small_table LEFT JOIN large_table"?
      1. USNikola Novak
 

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