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  1. PO
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    <p>The left join thing is a red herring.</p> <p>It is however an actual problem of running out of space for temp tables. But the order of your join makes no difference. The only thing that matters is how many rows MySQL must work with.</p> <p>Which brings me to the LIMIT command:</p> <p>Here is the problem:</p> <p>It order to get the single row you asked for, MySQL has to sort the ENTIRE record set, then grab the top one. In order to sort it, it must store it - either in memory or on disk. And that's where you are running out of space. Every single column you requested get stored on disk, for the entire table, then sorted.</p> <p>This is slow, very slow, and uses a lot of disk space.</p> <p>Solutions:</p> <p>You want MySQL to be able to use an index for the sorting. But in your query it can't. It's using an index for the join reference, and MySQL can only use one index per query.</p> <p>Do you even have an index on the sort column? Try that first.</p> <p>Another option is to do a separate query, where you select just the ID of the large table, LIMIT 1. Then the temporary table is MUCH smaller since all it has are IDs without all the other columns.</p> <p>Once you know the ID then retrieve all the columns you need directly from the tables. You can do this in one shot with a subquery. If you post your query I could rewrite it to show you, but it's basically <code>ID = (SELECT ID FROM ..... LIMIT 1)</code></p>
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    1. POIs it bad practice to do "small_table LEFT JOIN large_table"?
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    1. USAriel
    1. USAriel
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    1. POIs it bad practice to do "small_table LEFT JOIN large_table"?
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    1. COLike I said, I don't administer the database and I can't add indexes. I have only the read permissions. Also, the order of the join DOES make a difference. The query doesn't result in error, but the problem then is that if a device has made no records in large_table, then there is nothing to left join onto the data in small_table which does, in fact, contain a record of such a device. The admin told me to do that in two separate queries, but doing it that way just complicates applications which use the database unnecessarily IMHO. Also, I've updated my question to add the query.
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      1. USNikola Novak
    2. COIt works without the LEFT not because there is some issue with left joins, but because you are sorting on a column in the right table. But what happens if the right table is blank (missing)? How should it sort? What happens is that it's not able to just look in the large table and directly find the most recent date - that might not be the correct row, since it's possible the correct row will have no record in the large table, and only in the small table. So it must actually calculate everything, and then sort.
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      1. USAriel
    3. COSo tell me, what do you expect for blank rows in the large table? How should it sort? Why do you need the left join? You'll never actually get record from the small table, that are missing a record on the large table, because you are sorting by a value on the large table and getting only one of them. Missing rows will never show up.
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